The invention relates to a synergistic antibiotic composition useful for the treatment of respiratory, gastrointestinal or urinary infections and septicaemia of domestic animals. The composition comprises tiamulin hydrogen fumarate and an aminoglycoside antibiotic or a pharmaceutically acceptable salt thereof in a weight ratio of 5:1 to 1:5. The active components are admixed or diluted with a carrier, used in veterinary therapy, in a weight ratio of 1:1 to 1:50 and formulated for oral or parenteral application.