Glucocorticoids (GCs) have been considered to regulate immune cell systems through induction of apoptosis in thymocytes and mature peripheral-blood lymphocytes. Here we report that apoptosis induced by cortisol in mitogen-activated peripheral-blood mononuclear cells (PBMC) is suppressed by cortisone, an oxidized metabolite of cortisol. Apoptosis in PBMCs is quantified by a cell death ELISA procedure, which can specifically detect fragmented DNA. Cortisol induced PBMC-apoptosis at concentrations more than 10 ng/ml (28 nM) in concanavalin A-stimulated PBMCs and cortisone suppressed this apoptosis at a concentration range of 1-10,000 ng/ml (2.8-28,000 nM) dose-dependently. Prednisone, a synthetic oxidized-GC, also suppressed the apoptosis-inducing effect of cortisol in a dose-dependent manner. Suppression of cortisol-induced apoptosis by cortisone was consistently observed in PBMCs derived from 16 healthy subjects. Examination for inhibitory activities of the steroids against [3H]dexamethasone binding to PBMCs suggested that cortisone can bind cellular GC-receptors (GC-Rs), but the affinity of cortisone to GCRs is 1/30 or less than that of cortisol. The results raised a possible role of cortisone in cortisol-mediated regulation of apoptosis in activated human PBMCs. The counteracting action of cortisone against cortisol-induced apoptosis may take place partially through intervention of GC-receptors (GC-Rs), but may also be due to unknown pathway(s) different from those mediated by cellular GC-Rs.