The plaque flora isolated from discrete dentogingival sites during a human gingivitis experiment was analyzed as a function of the plaque score and of the gingivitis score. When the gingivitis score was plotted as a function of the plaque score, a nonbleeding gingivitis was associated with a proportional increase in the Actinomyces sp. at the expense of the Streptococcus sp. In particular, the percentage of Actinomyces israelii increased significantly, while the percent Streptococcus sanguis decreased significantly. A. israelii also increased significantly when a bleeding gingivitis developed. When the plaque score was plotted as a function of the gingivitis score, A. israelii increased significantly as the nonbleeding gingivitis developed, but A. viscosus and Bacteroides melaninogenicus increased significantly when the bleeding gingivitis developed. The availability of a sufficient number of plaques with a plaque score of 2.0 permitted the examination of the interrelationship of gingivitis and flora minus the effect of plaque biomass. The bacteriological profile showed that when bleeding occurred, the levels and proportions of A. viscosus and B. melaninogenicus increased significantly. These findings raise the possibility that proportional changes in the gingival plaque flora may uniquely contribute to the development of gingival inflammation in this experimental model.