Lactic and volatile acids in standardized plaque samples from 11 subjects have been estimated by means of HPLC before and after exposure to sucrose in vivo. Total acid differed considerably between subjects, but the ratio of acids estimated within the plaque from different persons did not vary significantly. The time-dependent acid pattern following sucrose rinsing allowed the plaque samples to be divided into two groups: one showing only small changes in total acid production; and the other exhibiting a distinct increase in total acid up to 15 min after sucrose rinsing.