Virus-related oncogenes have been demonstrated in human tumor cells and may play a role in neoplastic transformation. Cancerous effusions contain inhibitors of monocyte function and are absorbed by monoclonal antibodies to the immunosuppressive retroviral structural protein, P15E. We therefore examined eight human malignant cell lines for P15E-related antigens, by indirect immunofluorescence. Up to 87% of fixed malignant cells were reactive with two different monoclonal anti-P15E antibodies, while under identical conditions approximately 7% of freshly isolated human mononuclear cells were positive. Differentiation of two tumor cell lines with dibutyryl cyclic AMP resulted in decreased anti-P15E reactivity. Blast transformation of human mononuclear cells with mitogens induced reactivity with anti-P15E. Thus human malignant and blast-transformed cells contain antigens related to P15E. Expression of this viral-related gene may occur during rapid cell division and be abnormally regulated in cancer cells, thus rendering them more resistant to immune destruction.