The indigenous oral flora of 27 volunteers was monitored longitudinally over a 4-week period. Bacteria attached on buccal epithelial cells were counted by microscopy. Salivary bacterial colonies and the presence of alpha-hemolysis were examined after aerobic culturing on blood agar plates. The buccal and salivary bacterial counts were stably maintained in most subjects in the two repeated base-line samplings taken at 1-week intervals. Rinsing with a chlorhexidine mouthwash 45 min before sampling dramatically reduced the amount of epithelial cell-adherent bacteria. One day after the chlorhexidine rinse, however, the numbers of the epithelial cell-adherent bacteria exceeded the base-line level, and a similar decrease-increase pattern of changes was detected for the salivary alpha-hemolytic streptococcal counts. The non-hemolytic salivary bacterial counts were not affected by chlorhexidine. Subsequent weekly samplings showed no difference from the base-line samplings. The chlorhexidine-induced, delayed increase of viridans streptococci on oral epithelial surfaces should be considered a possible risk factor in medically compromised patients.