We have previously shown that strains of Eubacterium yurii are hydrophobic, as compared with human polymorphonuclear leukocytes (PMNs), possibly because of a crystalline surface layer (S-layer) covering the cell envelope of this potential endo-perio pathogen. The aim of the present study was to investigate the phagocytic ingestion by PMNs of the three E. yurii subspecies, with special attention to bacterial surface structures and hydrophobicity. Type strains of subspp. margaretiae, yurii, and schtitka, together with three clinical isolates from necrotic root canals, were studied. All strains were hydrophobic when tested by a two-phase partition method. E. yurii subspp. margaretiae strains ATCC43715T, ES4C, and ES14B-8E were resistant to PMN ingestion in the absence of opsonins, whereas strains of the two other subspecies were readily ingested. The presence of a resistant strain (subsp. margaretiae ATCC43715T) did not inhibit the ingestion of a sensitive strain (subsp. schtitka ATCC43716T). Ingestion of E. yurii subsp. margaretiae strains required opsonization by normal human serum or specific antibodies. Electron microscopy revealed an S-layer in all strains and fimbria-like structures in the subspp. margaretiae and yurii strains. The antiserum prepared against the S-protein of E. yurii subsp. margaretiae ATCC43715T showed only slight cross-reactivity with other E. yurii strains and indicated the presence of strain-specific rather than species- or subspecies-specific antigens in the S-protein of E. yurii subsp. margaretiae ATCC43715T. The results suggest that the mere presence of the S-layer or fimbria-like structures cannot explain the susceptibility to ingestion by the PMNs.(ABSTRACT TRUNCATED AT 250 WORDS)