We obtained clinical isolates of Porphyromonas gingivalis of known ribotype from patients diagnosed with adult periodontitis and used Western blot methodology to evaluate profiles of antigens recognized by IgG in heterologous and homologous patient sera. Our aims were to identify isolates belonging to different serogroups, to learn if serogroup membership is related to ribotype to assess variation in IgG responses of patients to antigens is homologous and heterologous ribotypes, and to determine the frequency of shared and variable antigens in different biochemical classes recognized across different serogroups and ribotypes. Blots of separation patterns of 28 isolates were developed in sera from patients and bound IgG was quantified by digital image densitometry. The membership of isolates in different serogroups was determined by correlation and hierarchical cluster analysis of isolate whole-cell IgG binding profiles. Two major isolate clusters, each with two subclusters, were found. Isolates within the same ribotype clustered together in some cases but not others. Homologous isolates ranked high in IgG binding levels relative to those from different patients irrespective of ribotype. Patient subgroups with IgG responses dominant for different ribotypes and serogroups were revealed by correlation analysis. The IgG binding profiles observed for individual protein and proteinase-resistant antigens across both homologous and heterologous isolates were very dissimilar. Furthermore, the frequency of antigens both shared across all ribotypes and recognized by IgG in patient sera was unexpectedly low. Only two protein antigens (Mr 44 kDa and 27 kDa) were strongly recognized across all ribotypes by different sera. We conclude that the IgG response of patients infected with a particular P. gingivalis serotype or ribotype is directed mainly against antigens that are not shared by other potentially infective clonal types.