Porphyromonas gingivalis is a periodontal pathogen that may also be involved in the pathogenesis of coronary heart disease. This microorganism has the ability to invade several cell lines. In this study, 26 different strains of P. gingivalis were tested for invasion of human umbilical vein endothelial cells and KB cells, a human oral epidermoid cell line. Abilities to invade both cell lines by an individual strain were similar, and their invasion efficiencies could be assembled into four groups: high, moderate, low and non-invasive. Of the 26 strains, only P. gingivalis AJW4 was non-invasive. Since the fimbriae are implicated as having a key role in invasion by this species, the presence of fimbriae on strain AJW4 was investigated. Using polymerase chain reaction (PCR), strain AJW4 was found to contain the fimA gene. Sequence analysis revealed it to be type IV according to the typing scheme developed by Amano et al. Further, fimA is transcribed in this strain as demonstrated by reverse transcription PCR and is expressed on the cell surface as visualized by negative staining and electron microscopy. The adherence+invasion of strain AJW4 was 38.7% of the most invasive strain (strain 381). However, the CFU ml(-1) of strain AJW4 recovered from within cells was 2.9% of strain 381. Even though strains AJW4 and W50 have the same type IV fimbriae, strain AJW4 is 8.9-fold more adhesive yet is internalized 170-fold less. These data indicate that the invasion efficiency of P. gingivalis is variable among the different strains, and that the expression of FimA is not sufficient for invasion.