Our efforts have been concerned primarily with cell mediated immune responses to the two human retroviruses causing diseases in man, namely HTLV-1 and HIV. We have demonstrated that chronic infection with either virus may result in high levels of circulating cytotoxic T cells (CTL) that can be detected in freshly separated peripheral blood mononuclear cells (PBMC). These CTL are CD8+ and MHC Class I restricted. In individuals infected with HIV, CTL activity is found more often in healthy patients with stable CD4+ counts, particularly in the subset of these patients with detectable levels of p24 antigen in their sera. CTL activity appears to decline with clinical progression to AIDS. In contrast, in patients infected with HTLV-1, high levels of these circulating CTL are only found in patients with neurological disorders, particularly individuals with tropical spastic paraparesis (TSP). CTL in HIV-1 infected individuals are primarily generated against structural proteins, while in patients with TSP, the predominant response is against products of the pX region. Using limiting dilution cloning techniques and mitogenic stimulation, we have generated HIV-1 and HTLV-1 specific CTL clones. Using synthetic peptides, we have mapped two new CTL epitopes within HIV-1 gag and nef proteins. We have demonstrated that CTL epitopes to HIV-1 proteins can be mapped directly using freshly separated PBMC. We have also shown that a synthetic peptide containing the correct epitope may not be recognized by CTL due to surrounding amino acid sequences that may influence binding to HLA Class I and recognition by the T cell receptor. We have noted that CTL to nef can lyse HIV infected cells and such clones can inhibit HIV replication in vitro. We have found that if these clones are activated with peptides containing the appropriate epitope, inhibition of viral replication can occur through the release of soluble products from these cells, obviating the need for contact with CD4+ cells harboring HIV. We have also defined a method for mapping B cell epitopes that are recognized by antibodies mediating antibody-dependent cellular cytotoxic responses (ADCC) and have shown that high levels of circulating CD16+ cells are armed with these antibodies in vivo.