We investigated the action of cholera toxin on the intracellular ionized calcium [Ca2+]i increase induced by anti-CD2 and anti-CD3 monoclonal antibodies in the leukemic human T-cell line Jurkat. Cholera toxin inhibits in a dose-dependent manner these two pathways of human T-lymphocyte activation but with different half maximal inhibition doses (75 n g / d for CD3, 30 n g / d for CD2). This effect cannot be accounted for only by the increase in cAMP induced by cholera toxin because forskolin, which raises cellular cyclic adenosine monophosphate (CAMP) to the same levels, induced only a small inhibition of the [Ca2+]i increase in similar conditions. Cholera toxin induced a decrease in the surface expression of the CD3 molecule, suggesting a down-regulation of the CD3 molecules. On the other hand, the expression of CD2 remained unchanged. Cell surface disappearance of the CD3 molecule cannot account for all the inhibitory effects of cholera toxin because CD2 molecule expression was not affected (no modifications in the half maximal binding of anti-CD2 monoclonal antibodies). All together, these results suggest that cholera toxin acts on substrates, possibly G proteins, that could regulate the [Ca2+]i increase induced by antLCD2 and antLCD3 mAbs in Jurkat cells. In addition, the present study demonstrated that the rise in cellular cAMP partially inhibits the [Ca*+]i increase induced by anti-CD2 and anti-CD3 mAbs .