Mononuclear cells are activated during hemodialysis. In this study, we provide evidence that in vitro culture of mononuclear cells with Cuprophan, a nonbiocompatible hemodialysis membrane, increases the levels of protein phosphorylation in these cells as well as the expression of surface activation molecules. By contrast, culturing of mononuclear cells with AN69, a more biocompatible membrane, did not increase protein phosphorylation levels or expression of surface activation molecules in these cells. In addition, Cuprophan, but not the AN69 membrane, increased the percentage of mononuclear cell death by apoptosis. Inhibition of G-protein-mediated signal transduction decreased the apoptosis of cells cultured with the Cuprophan membrane. The GTP-binding protein involved in Cuprophan-induced apoptosis was sensitive to the ADP-ribosylating pertussis toxin (PTX). The inhibition of GTP-binding protein decreased apoptosis in the early stage of the activation-induced apoptosis, suggesting that G-proteins are implicated in the transmission of apoptosis-inducing signal(s) but do not interfere with the effector signals that mediate the late stages of apoptotic catabolism. Finally, PTX was capable of inhibiting apoptosis without affecting expression of activation molecules; thus, the inhibition of apoptosis by Cuprophan was not due to quenching of the stimulation signals, because monocytes were still able to be activated by Cuprophan despite the action of PTX. The results obtained in this study suggest that cell activation and apoptosis may be mediated by separate intracellular signals.