Tiamulin concentrations have been determined in the serum of chickens, turkeys and swine after a single oral administration of various doses. The compound peaked between 2 and 4 hours after drug supply. In the highest dose used (50 mg/kg body weight), mean concentrations of 3.5, 2.9, 4.5μg/ml for chickens, turkeys and swine, respectively, could be obtained. In poultry, the levels declined between 12 and 24 hours to values not useful for calculation. In swine at 24 hours after administration measurable values could still be detected.