The ability of different Porphyromonas gingivalis strains (15 clinical isolates and ATCC 33277) to attach to and invade KB cells, in relation to other properties such as release of interleukin (IL)-6 and IL-8, cytotoxicity, proteolytic activity and types of fimbriae genes present, was examined. A hierarchical cluster analysis based on adherence and internalization resulted in four groups. Eight of the 15 clinical isolates belonged to a cluster group whose adherence and internalization were about 10% those of the ATCC strain. A negative correlation between lysine-specific protease activity and adherence was found. In all cases the released concentrations of IL-6 and IL-8 were very low. Only one strain was found to be cytotoxic to KB cells. Principal components analysis demonstrated correlations between adherence, internalization and autoaggregation. Most strains had fimA type I and II, type I being associated with elastase-like activity. The ability of P. gingivalis to invade epithelial cells may be a key factor for maintaining periodontal disease.