This study describes the predominant early microflora on human teeth on the basis of microbiologic identification of 1742 fresh isolates. The isolates were obtained from four dental students who carried test pieces of enamel and root surface in the oral cavity for 4, 8, 12, and 24 h. During the experimental periods oral hygiene was discontinued. Under equal conditions root surfaces were more heavily colonized than were enamel surfaces. However, the composition of the microbiota was the same. Within the first 24 h the microflora was dominated by streptococci and Gram-positive pleomorphic rods. S. sanguis contributed only 6-18% of the early colonizers whereas S. mitis and S. oralis varied between 24-42% and 1-27% (mean values), respectively. The relative proportion of S. oralis increased significantly within the observation period while the proportion of S. salivarius and arginine-positive S. mitis showed a declining tendency. Actinomyces species adsorbed to the tooth surfaces within the first 4 h but did not increase their relative proportions until after 8-12 h, possibly due to a long doubling time. In one individual, encapsulated bacteria resembling Stomatococcus mucilaginosus were observed among the early colonizers. The time-dependent shifts in the bacterial populations within 24 h corroborate parallel ultrastructural findings.