The effects of a thymic hormone (Facteur thymique serique; FTS) on renal reactive oxygen species-scavenging enzymes or substances in heminephrectomized rats with and without tacrolimus-induced nephrotoxicity were studied. Rats received both oral dose of tacrolimus (5 mg/kg/day) and subcutaneous administration of three dosages of FTS (5, 50, and 250 microg/kg/day) over 28 days (Group A). In Group B, they received three dosages of FTS alone (0.5, 5, and 50 microg/kg/day) or FTS 50 microg/kg/day with tacrolimus over 28 days. Each dose of FTS (Group A) partially elevated renal creatinine clearances. Tacrolimus enhanced renal glutathione reductase (GSH-R) activities and glutathione (GSH) and depressed catalase (CAT) activities. FTS increased GSH levels and GSH-R activities. Although FTS alone did not change CAT activities, CAT activities recovered as a result of concomitant use of FTS (Groups A and B). A significant positive correlation was found between CAT activity and creatinine clearance. These findings suggest that FTS is useful for the prevention of tacrolimus-induced nephrotoxicity, and that the increase of renal CAT activity in the defense mechanism of FTS might be important for cell protection against active oxygen species.