Abstract The human CD8 glycoprotein regulates the function of cytotoxic T cells activated by antigenic peptide as well as via CD3 or CD2 mAbs. Activation of T cells by CD2 mAbs requires two mAbs directed against distinct CD2 epitopes and induces tyrosine phosphorylation, PI-PLC activity generating the second messengers, IP3 and DAG, and finally lymphokine secretion. We have investigated the role of the CD8α molecule in CD2-mediated activation of human cytotoxic T cell clones and CD8 + resting T cells. CD8α-specific mAb inhibited 60% of the allospecific cytotoxicity of the CD8 + clone against its target cell and 86% of the CD2-redirected killing against the HLA Class 1-negative Daudi target cell. In addition. CD8α-specific mAb inhibited CD2-mediated TNFα and IL2 secretion by the CD8 + clone. Furthermore, CD8α,-specific mAb inhibited the increase in intracellular ionized calcium mediated by CD2 mAbs in the CD8 + clone and in purified T cells. Since the [Ca 2+] i recruitment from internal stores induced by CD2 mAbs was inhibited, the inhibitory effect induced by the CD8α-specific mAb probably acts on the PI-PLC activation pathway. This inhibition mechanism involves neither a decrease in affinity of CD2 mAb for its target nor a decrease in CD2 cell surface expression or a rise in cAMP known as an inhibitor of the CD2-mediated PI-PLC activity. These results suggest that the inhibitory mechanism induced by the CD8 mAb may prevent the activation of the PI-PLC activity, probably through the CD8α-associated protein tyrosine kinase p56 kk.