The chronic intravenous toxicity of cefodizime sodium (THR-221) was studied in beagle dogs. Groups of 6 males and 6 females were treated with THR-221 at doses of 0 (saline), 200, 400, 800 and 1600 mg/kg/day for 6 months. The THR-221 related symptoms were vomiting, excessive drinking behavior and salivation. The paleness of the visible mucosa and discoloration of vascular color by funduscopy due to systemic anemia were observed in one animal each of 800 and 1600 mg/kg/day groups. Body weight was depressed transiently or continuously in a few animals of 400-1600 mg/kg/day groups. The hematological, serum chemical and urinalysis findings in a few animals of 400-1600 mg/kg/day groups revealed decreases in RBC count, PCV and hemoglobin, an increase in reticulocyte count, a decrease in WBC count, a decrease in platelet count, slight increase in TP, and albumin, a decrease in AlP, and an increase in urinary Na. Light microscopically, deposition of hemosiderin and increased extramedullary hematopoiesis in the liver and spleen, and deposition of fibroid substance in the white pulp of the spleen and diffuse fibrosis in the bone marrows were detected in a few animals of 800 and 1600 mg/kg/day groups. Electron microscopically, no significant toxic changes were observed. The maximum nontoxic doses of THR-221 are estimated as 200 mg/kg/day in male and less than 200 mg/kg/day in female.