The effects of glucocorticoids administered in vivo and in vitro on lectin-induced proliferation of lymphocytes sampled from venous blood were investigated in patients with atopic dermatitis (AD) and in normal controls. Stimulation by concanavalin A (Con A), phytohaemagglutinin A (PHA) and pokeweed mitogen (PWM) in patients and controls did not differ significantly under baseline conditions. After in vivo administration of methylprednisolone the decline of Con A-induced blastogenesis of leucocytes was similar in both groups, whereas PHA stimulation caused a significant reduction in the controls only. In vitro addition of different dexamethasone concentrations had a pronounced suppressive effect on Con A- and PHA-induced blastogenesis in both groups, whereas PWM stimulation was unaffected. Pretreatment in vivo with methylprednisolone further decreased the suppression of the Con A and PHA lymphocyte proliferation rate by dexamethasone added in vitro in controls but not in patients. With regard to B-cell proliferation generated by PWM, no consistent glucocorticoid effect could be observed. The impaired effect on lymphocyte blastogenesis of glucocorticoids administered in vivo, in contrast to a normal in vitro reaction to dexamethasone, together with recent findings of an altered glucocorticoid receptor pharmacology in AD, points to a decreased biological in vivo efficiency of methylprednisolone in atopic dermatitis.