A monkey virus linked to human cancers may have contaminated the oral polio vaccine for years after the U.S. government ordered manufacturers to remove it. The Chronicle reported last week that the simian virus SV40 had contaminated early polio vaccine given to millions of Americans. When health officials discovered in 1961 that SV40 caused malignant tumors in lab animals, they ordered the virus eliminated from all future vaccine. But internal memos from Lederle Laboratories, the chief producer of polio vaccine in the United States, indicate SV40 may not have been completely removed. According to one memo, SV40 was found in three of 15 lots of the oral vaccine seven months after the federal directive was issued in March 1961. Lederle released the contaminated vaccine to the public anyway, the memo shows. Scientists discovered SV40 in the Salk polio vaccine in 1960. By then as many as 30 million Americans had been given injections of the SV40-tainted polio vaccine, which was first licensed in 1955. In recent years more than 60 scientific studies have found SV40 in rare human brain, bone and lung-related cancers, the same kinds of tumors the virus caused in laboratory animals. Some scientists believe SV40 may play a role in causing those cancers. The Lederle documents, which were obtained by Philadelphia attorney Stanley Kops in litigation not related to SV40, raise the possibility the virus might have been transmitted by contaminated oral vaccine, licensed for production in 1962.