Bacteriophages lytic for Salmonella typhimurium were isolated in considerable numbers from chickens experimentally infected with S. typhimurium, and in much lower numbers from the chicken feed. Lytic phages were also regularly isolated from human sewerage systems. One of these was used to inoculate S. typhimurium--infected two day-old chickens orally and via the feed. The phage took longer to establish in the caeca than did the Salmonella and it disappeared when the caecal S. typhimurium counts fell to 10(6) CFU/ml. No neutralizing antibodies to the phage were detected in the serum of these chickens. In a second experiment, five of 30 chickens similarly infected with S. typhimurium were inoculated with the phage. Within 3 days, the phage was isolated from 72% of the "in-contact" birds. A second phage, isolated from sewage, when inoculated into newly-hatched chickens simultaneously with any of 3 strains of S. typhimurium, produced a considerable reduction in mortality in the birds. This effect was only produced by inoculation of high concentrations of phage (greater than 10(10) PFU/ml). The phage produced reductions in the viable numbers of S. typhimurium in the crop, small intestine and caeca for up to 12 h after inoculation, with smaller reductions in bacterial numbers in the liver at 24 and 48 h after infection.