We investigated the bactericidal and bacteriostatic effects of yogurt on three strains of Escherichia coli: human toxigenic (078:H11), rabbit pathogenic (RDEC-1) and rabbit nonpathogenic [015:K14(L):H4]. Approximately 106 organisms were incubated in yogurt, milk, broth, and modifications of these materials. Aliquots were removed at various intervals and plated on MacConkey's agar for enumeration of E. coli. Yogurt was bactericidal (at least 5 log10 reduction in bacterial counts) to all three strains of E. coli with <10 CFU/ml remaining by 9 hr. In contrast, all three strains replicated rapidly in milk and broth, reaching maximum concentrations by 9 hr. The E. coli strains survived and multiplied in milk acidified to the same pH as the yogurt. Yogurt (native pH 4.1-4.4) in which the pH was brought up to and maintained at pH 5.5 or pH 7 for 8 hr was not bactericidal to E. coli Heat-treated yogurt and the filtered supernatant of centrifuged yogurt (both containing no yogurt bacteria) were only bacteriostatic. We conclude that both live yogurt bacteria and a pH near 4.5 are necessary for the bactericidal activity of yogurt. The possibility that yogurt ingestion could protect against infection via other foods contaminated with pathogenic E. coli merits further in vivo investigation.