Serum level studies in cats and dogs dosed with 30 mg per kg per day of either oral or injectable formulations of a 1:5 trimethoprim/sulphadiazine combination showed good absorption by both routes of administration. Toxicity tests with the products demonstrated a high margin of safety in dogs given this dose level; the margin of safety in cats was lower but still adequate. The high success rate in clinical trials against a wide range of bacterial infections confirmed the predictions from the serum level results that a dose rate of 30 mg per kg per day orally or parentally, would be therapeutically efficacious in both species.