Methisoprinol is an antiviral drug with immunopotentiating properties. The aim of this study was to assess the immunopharmacokinetic profile of the drug alone or administered in conjunction with corticosteroids. The plasma of patients who had been given a single dose of methisoprinol, prednisolone or both was collected before the infusion and at different time intervals thereafter. The plasma samples were tested for immunosuppressive or immunopotentiating activity in a mixed lymphocyte culture (MLC). The test was modified to increase sensitivity and decrease variability. The administration of methisoprinol resulted in increased responses up to three hours after the infusion. The simultaneous administration of both drugs resulted in an even more marked inhibition of MLCs as compared with corticosteroids alone.