An assay is described for evaluating live-culture treatment material that may be given orally to chicks to prevent intestinal colonization by non-host-specific salmonellae. Both pre-treated and control chicks are challenged with ca 104 salmonellae/chick, using a strain bearing an antibiotic resistance marker. Chicks are examined 5 d after challenge to determine both the proportion of positive birds in treated and control groups and the levels of Salmonella in the caeca of infected individuals. The efficacy of the treatment is determined by calculation of values for Infection Factor and Protection Factor.