The immunosuppressive and toxic properties of the recently discovered macrolide antibiotic FK506 were examined in comparison and in conjunction with cyclosporine administration in the rat. Male Sprague-Dawley rats were immunized systemically with sheep erythrocytes and received, from the same time, either FK506 (1 mg/kg/day) intramuscularly or CsA (25 mg/kg/day) by gavage, or both drugs in combination. Seven days after immunization, the splenic plaque-forming cell response and circulating antibody titers were reduced greater than 90% in animals receiving either FK506 or CsA and in the drug combination group. These immunosuppressive effects of FK506 and CsA were accompanied by significant increases in the incidences of splenic OX-8+ cells and by corresponding reductions in the W3/25+:OX-8+ ratio. No further changes in T cell populations were observed in animals given both drugs. A progressive monocytosis was found in response to CsA, but not in FK506-treated rats. Increases in plasma urea were observed in FK-506 and drug-combination or CsA-treated rats on day 7, whereas creatinine levels were raised only in the FK-506 groups. Elevated bilirubin levels and alterations in liver enzyme activities were observed in CsA-treated rats by day 4, whereas FK-506 alone produced no similar effects. CsA-treated rats also exhibited elevated blood and urinary glucose levels from day 4. No biochemical evidence of additive drug toxicity was detected. The only histological abnormalities observed were thymic medullary atrophy in all drug-treated animals, together with very minor reductions in bone marrow cellularity in a proportion of those rats given FK-506. These findings show that, at the dosage selected, the powerful immunosuppressive activities of FK-506 were associated with little evidence of acute toxicity and with no indications of additive toxicity with CsA.