A heat-labile sonic extract of a nonleukotoxic strain of Actinobacillus actinomycetemcomitans caused a dose-dependent inhibition of proliferation of human keratinocytes. Exposure of cells to heat-labile sonic extract for periods as brief as 10 min produced significant suppression of [3H]thymidine uptake 72 h later. I propose that alteration of epithelial barriers may be one pathogenic mechanism...