Developing denial bacterial plaques formed in vivo on enamel surfaces were examined in specimens from 18 adult volunteers during the first day of plaque formation. An intraoral model placing enamel pieces onto teeth was used to study bacterial plaque populations developing naturally to various cell densities per square millimeter of surface area of the enamel (W. F. Liljemark, C. G. Bloomquist, C. L. Bandt, B. L. Philstrom, J. E. Hinrichs, and L. F. Wolff, Oral Microbiol. Immunol. 8:5-15, 1993). Radiolabeled nucleoside incorporation was used to measure DNA synthesis concurrent with the taking of standard viable cell counts of the plaque samples. Results showed that in vivo plaque formation began with the rapid adherence of bacteria until ca. 12 to 32% of the enamel's salivary pellicle was saturated (ca. 2.5 x 105 to 6.3 x 105 cells per mm2). The pioneer adherent species were predominantly those of the 'sanguis streptococci.' At the above-noted density, the bacteria present on the salivary pellicle incorporated low levels of radiolabeled nucleoside per viable cell. As bacterial numbers reached densities between 8.0 x 105 and 2.0 x 106 cells per mm2, there was a small increase in the incorporation of radiolabeled nucleosides per cell. At 2.5 x 106 to 4.0 x 106 cells per mm2 of enamel surface, there was a marked increase in the incorporation of radiolabeled nucleosides per cell which appeared to be cell-density dependent. The predominant species group in developing denial plaque films during density-dependent growth was the sanguis streptococci; however, most other species present showed similar patterns of increased DNA synthesis as the density noted above approached 2.5 x 106 to 4.0 x 106 cells per mm2.