The human T-cell lymphoma virus, HTLV-I, has been found to be associated with certain adult T-cell malignancies. Cell surface antigen profiles of HTLV-infected lymphocytes established in long-term cultures and a variety of cells infected by coculture methods were studied. The majority of cell lines established by coculture demonstrated the specific phenotype of OKT4, HLA-DR, the T-cell growth factor receptor, TAC, and an HLA-associated antigenic determinant that is detected by the monoclonal antibody, 4D12. A second but related retrovirus, HTLV-II, has also been studied under similar experimental circumstances. A cell line established from a patient with acquired immune deficiency syndrome demonstrated the presence of the HTLV-I retrovirus. This retrovirus in coculture experiments infected OKT4 cells, as well as some cells with the suppressor phenotype, OKT8. The kinetics of expression of the cell surface antigens, HLA-DR, 4D12, and TAC were studied. Short-term cultures of peripheral blood lymphocytes (tumor cells) from patients were established and examined for the expression of cell surface antigenic determinants. Within 6 hours, HLA-DR, 4D12, and TAC were expressed. In the short-term cultures, viral demethylation was not observed as opposed to established cell lines. Hamster cells were fused with an HTLV-I cell line, cloned, and examined for the expression of the 4D12 determinant, HLA antigens, and human chromosomes. HLA expression was always associated with chromosome 6; however, the 4D12 expression was only associated with the presence of a virus infection. The results suggest that the 4D12 epitope expression is a product of the HTLV. Two B-cell lines were established from patients with adult T-cell leukemia and found to be HTLV positive. Alteration in immune function was demonstrated by HTLV infection of T-cell clones that had restricted immune response potential.