Intramuscular administration of cefoperazone, tylosin or their combination for 5 successive days in their subtherapeutic doses did not produce any harmful effects on blood, liver and kidney of rabbits. Therapeutic or double therapeutic doses of the drugs and their combination produced a significant decrease in erythrocytes, leucocytes counts, Hb% and P.C.V.% with a significant prolonged prothrombin time. It also elicited a significant elevation of SGOT, SGPT and total bilirubin without effect on alkaline phosphatase level. Moreover, the drugs induced a significant elevation of blood urea nitrogen and creatinine without any effect on uric acid level.