In the present study we compared the effect of rapamycin to that of CsA on the in vitro responses of lectin (PHA), phorbol-ester (PMA) and Ca 2 + ionophore (ionomycin)-activated peripheral blood mononuclear cells by measuring the release of soluble IL-2R (sIL-2R), high levels of which have been detected in clinical syndromes characterized by an ongoing immune activation. PHA was the stimulant associated with high sIL-2R release, whereas ionomycin-induced sIL-2R release only exceeded the background response and the sensitivity of the ELISA kit. The highest sIL-2R release, however, was obtained when PMA was used in combination with either PHA or ionomycin. Rapamycin inhibited the release of sIL-2R in response to all activators, whereas CsA only abolished the ionomycin-induced sIL-2R release. In parallel experiments rapamycin inhibited cell proliferation in response to all stimulants with the exception of PMA/ionomycin, whereas CsA inhibited all proliferation. Our study clearly shows that for optimal sIL-2R release both Ca 2 + and protein kinase C-triggered signals are required and that rapamycin has a distinct advantage over CsA in inhibiting the release of sIL-2R, which has been shown to be a reliable marker of lymphocyte activation either in vivo or in vitro.