The in vitro lymphocyte response to mitogen stimulation and the in vitro lymphocyte sensitivity to glucocorticoid were examined in 72 kidney transplanted patients before and after administration of high or low doses of glucocorticoid. Administration of 10 mg of prednisone orally to 10 patients did not significantly change the mitogen responses or the lymphocyte sensitivity to methylprednisolone. Likewise, administration of 100-120 mg of methylprednisolone, intravenously to 62 patients did not significantly affect the mitogen responses, but, in contrast, the lymphocyte sensitivity to the immunosuppressive effect of methylprednisolone was clearly increased. This effect was highly significant in both cyclosporine A and azathioprine treated patients. These findings suggest that a change of the lymphocyte sensitivity to the immunosuppressive effect of methylprednisolone may occur after a high dose of glucocorticoid, anaesthesia and surgery, although no changes of the immunefunctions in vitro can be demonstrated by examining the mitogen response of the lymphocyte cultures. No relationship was found in the present study between the individual lymphocyte sensitivity to glucocorticoid and metabolic clearance rate.