Restriction fragment length polymorphisms (RFLPs) of two hemagglutinin loci were analyzed in 36 Porphyromonas gingivalis isolates from human and monkey origins using portions of hagA and hagB as probes. The P. gingivalis strains were differentiated into 9 RFLP groups based on the heterogeneity of the hagA locus and 10 different groups based on hybridization with hagB. Homology to hagA was detected in all human derived and all but three monkey derived strains. All P. gingivalis isolates exhibited DNA homologous to hagB. Multiple alleles of the hemagglutinin genes were detected for most P. gingivalis strains. No DNA homologous to either hemagglutinin gene could be detected in 6 other bacterial species tested. Serotyping and hemagglutination titers of each P. gingivalis isolate were obtained in an attempt to establish a correlation between these pheno-typic parameters and RFLP group. Although no correlations were found with these parameters, a correlation between RFLP group and invasiveness in the mouse abscess model was noted.