Bovine enamel blocks were covered by a bacterial cell mass and held in the mouths of human volunteers by a palatal appliance, to study the effects of acid production from endogenous bacterial substrate on the pH of the mass and on enamel demineralization. Variables were cells with and without endogenous substrate, rinsing with 10 per cent glucose or sucrose solution, or no rinsing and the type of organism. Acid production from endogenous substrate by Streptococcus mutans strain IB-1600, due, at least in part, to intracellular glycogen degradation, caused a marked and prolonged pH drop and significant enamel demineralization. A similar but less marked effect was obtained with a strain of Streptococcus mitis, Streptococcus sanguis and Actinomyces viscosus. Thus acid production from endogenous sources by plaque bacteria may be of importance in the aetiology of dental caries.