The ability of five ATCC bifidobacterial species to produce antimicrobial substances was investigated by testing the effects of spent bifidobacterial broths on the growth Escherichia coli in the thioglycollate medium. Such broths were most inhibitory if their pH was not readjusted to neutrality. When that was done, the inhibition ranged from 30 to 43%. Such inhibition of E. coli growth could be duplicated by a 3:2 aceticlactic acid mixture adjusted to neutral pH. It was concluded that no antibacterial substances other than acetic and lactic acids were produced by bifidobacterial strains used, and that the effects of these ubiquitous fermentation products, as well as pH effects, be taken into consideration before the existence of other antimicrobial factors is proposed.