Three variables were included in a study to determine their effect on the incidence of Salmonella typhimurium in broilers challenged at four days of age. Variables included the presence or absence of a feed additive, avoparcin; the use of new or used litter and the initiating dose of salmonella. Cloacal swabs were taken from approximately 600 chicks at weekly intervals for 45 days. At 10(4), 10(6) and 10(8) c.f.u./chick there was a direct association of challenge dose and the incidence of positive chicks for the first several weeks. Chicks raised on used litter showed an appreciable reduction in susceptibility to salmonella when compared to control animals on fresh litter. As the birds approached slaughter age, the influence of litter hygiene and challenge dose diminished under the conditions of this study. Avoparcin in the diet at 10 p.p.m. had no enhancing effect on salmonella shedding at any time during the 45-day sampling period. The implications of competitive exclusion are discussed.