Plaque samples from 10 different tooth surfaces of 10 schoolchildren with varied caries experience (DFS 10-33) were collected five times during 2.5 years. The samples were examined with an immunofluorescent technique for identification and enumeration of Streptococcus mutans serotypes c/e/f and d/g. At each sampling occasion the children were scored for caries. A stimulated saliva sample was also collected and the number of S. mutans per ml saliva was determined. The saliva level of S. mutans was shown to reflect the prevalence and proportion of this microorganism on the selected surfaces. Five surfaces carried S. mutans at each sampling. Four of these surfaces showed progressive cries S. mutans infection was also found to precede the development of incipient caries on four surfaces. Eighty percent of the surfaces that stayed sound were only transiently carriers of S. mutans in mainly very low numbers. Serotype c/e/f dominated in prevalence and proportion on the surfaces with a history of caries during the study.