A natural lactic fermentation of mixtures of water and whole flour of either maize or high-tannin sorghum was obtained either before or after cooking to a weaning gruel: The preparations had a final pH of about 3.8 (range 3.67 to 4.00) and a ratio of lactic acid to acetic acid of 9:1 (w/w). The growth of added (about 107 c.f.u./g gruel) Gram-negative intestinal pathogenic bacteria, enterotoxigenic Escherichia coli, Campylobacter jejuni, Shigella flexneri and Salmonella typhimurium, was strongly inhibited in the sour gruels, and the effect could primarily be explained by the low pH caused by the formation of lactic and acetic acids during the fermentation process. Of the added Gram-positive bacteria, Bacillus cereus and Staphylococcus aureus showed similar inhibited growth up to 7h after inoculation in the sour gruels. The strain of Staphylococcus, however, showed only a continued reduction in growth in the fermented gruel samples, which had a viable lactic bacteria culture indicating the presence of a bacteriocin. This implies that a low pH (< 4.0) alone is not sufficient to sustain the inhibition of the growth of Staphylococcus aureus. The survival studies were carried out at optimal temperatures for each respective enteropathogen. © 1992 Rapid Communications of Oxford Ltd.