To prevent or minimize mizoribine enterotoxicity in organ transplant recipients and to differentiate mizoribine enterotoxicity from other causes of enteritis, serum levels of mizoribine that produced subclinical and clinical signs of enterotoxicity were determined in the dog. When mizoribine was administered orally at 12-hr intervals, half the dogs studied showed clinical evidence of gastrointestinal disturbances (vomiting, diarrhea, and anorexia) without histopathologic signs of enterotoxicity. Using a 24-hr oral-dose schedule, clinical signs of gastrointestinal disturbances and histopathologic evidence (mucosal degeneration, crypt degeneration, and necrosis) of enterotoxicity were encountered when the mean 12-hr mizoribine serum level was 0.97 +/- 0.4 microgram/ml or greater. Histopathologic signs of enterotoxicity with repeated positive fecal occult blood assays and without clinical signs of gastrointestinal disturbances occurred when the mean 12-hr serum level was 0.53 +/- 0.17 microgram/ml or greater. Oral administration of cyclosporine did not exacerbate mizoribine enterotoxicity in the dog when administered with mizoribine at a dose that produced histopathologic signs of enterotoxicity.