"Partially purified extracts from 33 human tumors of several histologic types were used as competing antigens in a radioimmunoassay for the p30 protein of an endogenous primate type C virus (M7). Antigens immunologically related to the p30 protein of the M7 virus were detected in two tumors. Like viral p30 antigens previously identified in tissues of several other primates, the antigens found in human tumors crossreact with the p30 protein of the feline RD-114 virus but are unrelated by similar immunologic criteria to the p30 proteins of several other mammalian type C viruses. Gel filtration shows that most of the antigenic activity co-chromatographs with authentic p30 protein. These results, along with those showing nucleic acid sequences related to those of an endogenous primate type C virus in the DNA of human cells, make it clear that humans, like other primates, have type C viral sequences in their genome and can, in some circumstances, express at least one type C viral protein."