Induction of T cell anergy is thought to occur during activation in the absence of adequate costimulation. Here we demonstrate induction of anergy in a CD8 T cell clone by its cognate Ag in the presence of B7-1 and B7-2 costimulation. Primary activation of a CD28+CD8+ T cell clone by either human T cell lymphotrophic virus type I (HTLV-I) Tax11-19 peptide-pulsed EBV-transformed B cells, CD40L-stimulated B cells, or T cells was sufficient to induce complete unresponsiveness to a secondary Ag challenge. This was not caused by lack of B7 costimulation since the APCs expressed B7-1 and B7-2 and failed to induce anergy in an MBP peptide 84-102-reactive CD4 T cell clone. While anergic CD8 T cells did not proliferate, they retained their ability to lyse peptide-pulsed target cells. However, Ag stimulation failed to induce IL-2 mRNA transcription and IL-2 secretion, although immediate early tyrosine phosphorylation was normal and anti-CD3 cross-linking induced identical levels of CD40L expression in anergized and non-anergized CD8 T cells. Secondary Ag stimulation in the presence of exogenous IL-2, however, resulted in normal proliferative response. Moreover, while stimulation of CD8 T cells with PHA and B cells induced anergy, CD8 T cell stimulation with PHA and mononuclear cells failed to do so. In addition, the presence of mononuclear cells during the exposure of CD8 T cells to peptide-pulsed B cells prevented the induction of anergy. Together, our observations demonstrate that at least a subpopulation of CD8 T cells are anergized when costimulation is provided by B cells or T cells.