In this study, we describe the time course of the influence of a single oral dose of prednisolone (10 mg) in man on the proliferative response of peripheral blood lymphocytes in whole blood. The data were fitted to an integrated pharmacokinetic-pharmacodynamic model, relating the plasma concentrations of prednisolone to its effect. The determination of prednisolone-induced lymphocytopenia and monocytopenia in vivo and the assessment of the influence of varying lymphocyte and monocyte numbers on proliferative responses in vitro enabled us to calculate the relative contributions of several prednisolone-induced effects to the diminished lymphoproliferative responses in whole blood after administration of prednisolone in vivo. We demonstrate that the decrease of the proliferative response in whole blood lymphocyte cultures stimulated with a monoclonal antibody directed against CD3 is mainly determined by the time course of the prednisolone-induced lymphocytopenia. The time course of the monocytopenia and the relative changes in the CD4+ and CD8+ T-cell subpopulations induced by prednisolone and the direct inhibitory effect of the changing plasma concentrations of the drug also contribute to the decrease of aCD3-stimulated whole blood lymphocyte cultures to some extent. The decrease of the proliferative response in whole blood lymphocyte cultures stimulated with a horse anti-human lymphocyte serum closely followed the time course of the lymphocytopenia induced by prednisolone. However, this response remained decreased for a longer period of time than could be expected on the basis of the prednisolone-induced lymphocytopenia in vivo. A possible mechanism which might explain this discrepancy between the prednisolone-induced effects in vivo and in vitro will be discussed.