Representative experiments from work undertaken to develop a synergistic mixture of trimethoprim and sulphaquinoxaline for the preventive treatment of certain poultry diseases are described. Sulphaquinoxaline in the diet for four days was shown to achieve at least an 85 per cent higher blood level than nine other sulphonamides in chicks, and the efficacies of various trimethoprim/sulphaquinoxaline regimes in the diet or in the drinking water were demonstrated against pasteurellosis, colisepticaemia and five kinds of coccidiosis. Regimes for bacterial diseases were begun one day before infection but those for coccidial diseases were begun on the same day as infection or later. Overall, a total dose of 30 mg/kg bodyweight/day (trimethoprim/sulphaquinoxaline = 1:3) controlled these seven diseases. The same treatment was also shown to control sulphaquinoxaline-resistant strains of Escherichia coli and Eimeria acervulina. Although both drinking water and food were used for drug administration, twice the inclusion rate was required in food to that in water for equivalent efficacy. The significance of different modes of expression of dosages for bacterial and coccidial diseases is explained.