WASHINGTON — A Democratic presidential candidate, Senator Obama, said yesterday that he was not aware that he had invested in two companies backed by some of his top donors and did nothing to aid their business before the government.

The Illinois senator faced questions about more than $50,000 in investments that he made right after taking office in 2005 in two speculative companies, AVI Biopharma and Skyterra Communications. Mr. Obama said his broker bought the stocks as part of a quasi-blind trust.

“At no point did I know what stocks were held, and at no point did I direct how those stocks were invested,” Mr. Obama told reporters at the end of a press conference called to tout an unrelated immigration bill.

“What I wanted to make sure is that I didn’t want to invest in companies that potentially would create conflicts with my work here,” said Mr. Obama, who has campaigned on the need for stronger congressional ethics rules. “Obviously, the thing didn’t work the way I wanted it to.”

Mr. Obama purchased $5,000 in shares for AVI, which was developing a drug to treat avian flu. Two weeks after buying the stock, Mr. Obama pushed for more federal funding to fight the disease, but he said he did not discuss the matter with any company officials.

Mr. Obama also had more than $50,000 in shares of Skyterra, a company that had just received federal permission to create a nationwide wireless network that combined satellite and land-based communications systems.

Among the company’s top investors were donors who raised more than $150,000 for Mr. Obama’s political committees, the New York Times reported yesterday. The stock holdings were first examined Monday by the financial Web site, Thestreet.com.

The reports found no evidence that any of his actions ended up benefiting either company during the roughly eight months he owned the stocks. Mr. Obama lost about $15,000 on Skyterra and earned a profit of about $2,000 on AVI.