The vast majority of in vitro experiments testing the cytotoxic T lymphocytes (CTL) activity in HIV infection has been performed with target cells consisting of autologous EBV-transformed B lymphoblastoid cell lines (B-LCLs) expressing Human immunodeficiency virus type I (HIV-1) proteins. However data concerning the lysis of primary CD4 + T lymphocytes expressing HIV-1 antigens by CTLs is still lacking. To study the CTL activity against such primary targets, we used a system involving PBMCs of an HIV + asymptomatic patient (PT) as effector cells and the CD4 + lymphocytes or B-LCLs of his healthy HLA-identical twin brother (HTW) as target cells. These syngeneic targets were either infected with recombinant vaccinia virus containing HIV-1 gag gene (gag-vac), or coated with HIV-1 gag peptides. We demonstrate in this study that PT CTLs (which were CD3 + , CD4 − , CD8 + , TCR α β + , TCR γ δ − , CD56 − ) specifically lysed both types of syngeneic target cells expressing gag-vac; however, CD4 + T cells expressing HIV gag proteins were lysed less efficiently than B-LCLs expressing the same HIV epitopes. On the other hand, no specific lysis was detected when the target cells were uninfected or infected by wild-type vaccinia virus.