Although the inhibition of bacterial glycolysis by zinc ions might be expected to moderate dental caries, there has not been a comparison of the effect of Zn on different organisms under both fixed pH and free-fall conditions. Here, the effect of ZnCl(2) on Streptococcus salivarius, Strep. mutans, Strep. sobrinus, Actinomyces naeslundii and Lactobacillus casei, as well as on mixtures of oral organisms outgrown from human dental plaque and saliva, was surveyed. pH-stat experiments were performed at pH 7, 6 or 5 in a solution containing 5% glucose and a suspension of the test organism; pH-fall experiments started at pH 7. In both cases, acid production was monitored for 60 min, when samples were taken for Zn and lactate determinations. Under pH-stat conditions, acid production was inhibited by Zn most strongly in Strep. sobrinus and Strep. salivarius. In terms of total acid production averted, however, the effect of Zn under both pH-stat and pH-fall conditions was clearly greatest with Strep. salivarius. A. naeslundii was inhibited the least strongly under pH-stat conditions. Cultured oral organism mixtures were more sensitive to moderate concentrations of zinc (0.2-0.3mM initial concentration) than were the single species to higher concentrations (1mM). Packed cell layers responded to Zn quite differently from suspensions, the pH often falling in the presence of 1mM Zn at a rate similar to the no Zn control. As streptococci had the highest acidogenesis rates in both pH-stat and pH-fall experiments, it seems likely that inhibition of acid production with these organisms would be of more value in moderating caries than the inhibition of less acidogenic organisms such as A. naeslundii.