The minimal inhibitory concentrations (M1C) of tiamulin and tylosin for mycoplasma. Gram‐positive, and Gram‐negative micro‐organisms isolated from chickens were determinated by the agar dilution method. Median M1C values for tiamulin againstMycoplasma gallisepticum(0.05 μg/ml) andMycoplasma synoviae(0.10 μg/ml) were 2 to 4 times lower than the corresponding values for tylosin. Tiamulin was also slightly more effectivein vitroin inhibitingEscherichia coli, Pasteurella multocida,and beta‐haemolytic streptococci than was tylosin. Groups of chicken were offered tiamulin medicated drinking water at rates of 125 and 250 mg/litre for 48 hours. Average serum tiamulin concentrations were 0.38 and 0.78 μg/ml, respectively. When tylosin tartrate was added to the drinking water at 500 and 700 mg/litre, average serum drug levels were 0.12 and 0.17 μg/ml, respectively.