A 1996 adults-only installation by Paul McCarthy with a top estimate of $4 million was unsold at Phillips de Pury’s London auction last night, the priciest flop in a sale where one in three lots didn’t find buyers.

Mr. McCarthy’s “Bunk House,” featuring a dog-headed bartender and a bare-bottomed blonde in a Western setting, was one of 31 artworks that didn’t sell at Phillips, the first of a series of contemporary-art auctions in London this season that’s expected to fetch as much as 329 million pounds. Phillips’s auction last night totaled $48 million with fees, against its own presale estimate of $57 million to $80 million.

“This was regular material with very strong estimates,” Brussels dealer Paolo Vedovi said in an interview. “Too many sales, too many catalogs.”

Phillips’s director of contemporary art worldwide, Michael McGinnis, said after the sale he was “genuinely surprised” by the low percentage of lots sold.

“Sellers’ expectations are high and we did take some risks on some lots,” Mr. McGinnis said.

Phillips guaranteed 41 of the 91 lots offered last night, including the McCarthy. Twelve of the guaranteed lots failed to sell, representing a lower-estimate value of $10 million.

When auction houses guarantee items, they usually share a percentage of the “upside” above an agreed minimum price; if the lot fails to sell, the auction house — or a third-party guarantor — pays for the work and owns it.