The Keith Haring mural that has stood at Houston and Bowery streets since April is a tribute, an anniversary gift, and a historical re-creation. As of last week, it is also a battleground.

Last Friday, under the late morning sunshine, an artist named Angel Ortiz added thickets of graphic black lines in the spaces between Haring’s original green and black figurines. His was no random act of vandalism. Mr. Ortiz, who goes by the nickname LA II, was an early collaborator of Haring’s, working closely with the artist between 1980 and 1986. He has been credited by Haring in John Gruen’s “Keith Haring: The Authorized Biography” with advancing the Pop artist’s creative development, and has been called Haring’s “silent partner.”

In recent years, however, Mr. Ortiz has broken his silence. Mr. Ortiz’s additions to the East Village mural are now the latest chapter in a nearly 20-year dispute over artistic license, credit, and propriety.

Mr. Ortiz has accused the Haring Foundation of denying him credit on many of the jointly produced works and of withholding a trust fund Haring, who died of AIDS in 1990, set up in his name. (Mr. Ortiz was underage during the years he worked with Haring.) In an interview with The New York Sun, Mr. Ortiz said that the executive director of the Haring Foundation, Julia Gruen, John Gruen’s daughter, has repeatedly asked him to help authenticate Haring works in preparation for sale at auction houses such as Sotheby’s while refusing his request for recognition by the foundation. Ms. Gruen did not respond to multiple requests for comment.

The mural, which was commissioned by the Haring Foundation; Deitch Projects, which represents Haring’s estate, and the real estate developer Tony Goldman, who owns the wall on which it is painted, was produced in celebration of the 50th anniversary of Haring’s birth. Originally painted by Haring in the same location, the mural was re-created by a team of artists using photographic documentation by Haring’s friend Tseng Kwong Chi.

Shortly after its official unveiling in May, Mr. Ortiz received a call from a friend alerting him to the fluorescent artwork popping off a dingy stretch of East Houston. Mr. Ortiz turned to an artist and gallerist who has championed him in the past, Clayton Patterson, and asked him to help him tag the mural with his own artwork. “I thought it needed something to make it look better,” Mr. Ortiz said with a smirk.

“We said, ‘How do we sort of get some justice to his cause?'” Mr. Patterson said. “And really the most intelligent way of doing it was the wall.”

Mr. Ortiz, along with Mr. Patterson, spent an hour working on the mural, filling in the blank areas with squiggles and the words “LA II,” “Clayton,” and “LES.” The two claim that the removal of the fences that originally cordoned off the mural rendered the artwork public, justifying their alterations. “It gets that reminder going that this was a two-person thing,” Mr. Patterson said. “It’s not rude. It’s not violent. It’s not like a protest. It’s a very artistic way to enter into a dialogue.”

Neither Mr. Ortiz nor Mr. Patterson has received a response to their work from either the foundation or Deitch Projects.

Mr. Ortiz has found scattered success since the ’90s as an artist. His most recent show in America was at Mr. Patterson’s gallery in 2002. His work sold at auction at Phillips de Pury in April 2008, though for just $3,500. He currently has a solo show called “Milk Money” at the Follin Gallery in Italy.

Mr. Ortiz met Haring during the summer of 1980, when a mutual friend introduced the two at Haring’s request; Haring had seen Mr. Ortiz’s tag around the Lower East Side and East Village. “It stood out because it was absolutely perfect and beautiful,” Haring said in Mr. Gruen’s biography. “It was like finding another Jean-Michel,” he said, referring to Basquiat. “So of course, I wanted to meet this LA II … So, it’s this very young kid! He’s fourteen years old, he’s short, he’s totally adorable and sweet. We just immediately hit it off. It’s as if we’d known each other all our lives. He’s like my little brother. From the beginning, it’s a paternal or fraternal sort of relationship — that’s all it is.”

The two collaborated on an initial work, drawn on a discarded taxicab hood. After Haring sold the work for $1,400, he gave Mr. Ortiz half of the money. The two subsequently had several joint shows, including Haring’s first at the Tony Shafrazi Gallery. Mr. Ortiz arrived at an interview yesterday toting a bagful of evidence of his close relationship with Haring, including the promotional postcard for the Shafrazi show, which depicts the two artists posing in front of a wall covered with their markings. Haring, his arms crossed, wears a T-shirt showing a “radiant baby,” which would become his trademark. Mr. Ortiz, baby-faced, shirtless, and barefoot, wearing only a pair of athletic shorts, stands beside him, engaged and animated.

In the years following, the two had shows in New York, Tokyo, and Italy. The Haring Foundation owns at least seven works jointly credited to Haring and LA II and dated during this period, and has lent several of them to museums including the Whitney and the Guggenheim. It has not, however, put any of them up for sale, an act that could potentially prove lucrative for Mr. Ortiz.

But while Mr. Ortiz maintains that the foundation mismanaged the trust fund Haring arranged for him, his main dispute is regarding credit, not cash. “I was never interested in the money,” he said. “I just want her to say, ‘LA, we did you wrong,'” he said of Ms. Gruen. “That would be beautiful.”