Gingival biopsies of the mesial papilla area of the first molar were obtained from each patient at 0-, 14- and 21-day intervals during plaque formation. The biopsies were fixed, serially sectioned, and Gram-stained. The incidence and distribution of the bacteria-like structures were studied by microscopy. In all the specimens the bacterial nature of Gram-stained material was substantiated. In the epithelium the highest number of bacteria was found at the outer layer of marginal oral epithelium, sulcular epithelium and apical oral epithelium along with a decreasing pattern of penetration progressing deeper into the layers of tissue. For junctional epithelium the situation was just the opposite. Each subject had significantly higher counts at Day 21 than at Day 14 for both epithelium and connective tissue. Also significantly higher counts were found in connective tissue compared with epithelium. The higher bacterial density of intragingival bacteria was associated with the higher gingival and plaque indices. This study suggests that early stages of gingival inflammation may be mediated by invasion of bacteria.