The immunosuppressive effects of FR 900506 were investigated for one month after canine kidney and liver transplantation. Eight untreated kidney recipients lived for 8 to 19 days, mean 12.1. In contrast, the one month survival of dogs given FR 900506 in daily oral doses of 0.5, 1.0, and 1.5 mg/kg was 2/6, 3/6, and 4/6 respectively. The one month survival of liver recipients was 4/5 compared to one month survival of 1/8 in historical untreated controls. There was no evidence from biochemical tests of hepatotoxicity or nephrotoxicity.