An 8-month-old female Terrier and a 10-year-old female Pointer were referred to the University of Tehran Veterinary Teaching Hospital.

History of the first case was lethargy, weakness, vomiting and ataxia. Laboratory findings showed a significant neutrophilia( WBC,46250 cells/ μl ; neutrophils,41625 cells/μl; PCV, 29%; platelet count, within normal range and the result of its direct comb's test for detection of IMHA was negative) in first referring. This case was treated with Cotrimoxazole( single dose of 200 mg IV followed by oral suspension 200 mg/case bid for 10 days). Laboratory findings after 14 days were included: PCV, 21%; WBC, 1600 cells/μl; platelet, 60000/μl and its direct Coomb's test was positive.

History of the second case was ovariohysterectomy as a result of pyometra and treatment with Cotrimoxazole (oral suspension 400 mg/case bid for two weeks). Laboratory findings were included: PCV, 18%; WBC, 6000 cells/μl; thrombocytopenia and the result of its direct Coomb's test was positive.

Discussion: These results typically showed thrombocytopenia, leukopenia and anemia as a result of immune-mediated side effects of the use of Cotrimoxazole. In the light of these results the administration of Cotrimoxazole for a relatively long time should be avoided.