Previous investigators have suggested that opsonization of two Bacteroides species is mediated exclusively by the alternative complement pathway and requires immunoglobulins. In this study, the nature of the opsonic factors in nonimmune human serum for four species of Bacteroides was investigated by measuring uptake of [3H]thymidine-labeled bacteria by human polymorphonuclear leukocytes. Normal human serum, C2-deficient serum, immunoglobulindeficient serum, and serum chelated with ethylene glycol-bis(,f-aminoethyl ether)-N,N-tetraacetic acid (EGTA), MgEGTA, and ethylenediaminetetraacetic acid (EDTA) were used as opsonic sources. Heat inactivation of each of these sera significantly reduced its opsonic activity for all four Bacteroides species, suggesting that serum complement was essential for effective opsonization. All strains were opsonized in the absence of the classical complement pathway; however, kinetics studies revealed that opsonization proceeded at a significantly faster rate when the classical complement pathway was intact. Although two strains were opsonized in immunoglobulin-deficient sera, opsonization was less efficient and appeared to occur via the alternative complement pathway. Unexpectedly, all strains were well opsonized by the classical complement pathway in 10% serum which had been effectively chelated with EGTA or EDTA. The explanation for this finding is unknown; however, it is possible that cell wall cations of Bacteroides species may participate in the activation of complement in chelated serum, resulting in effective opsonization. It was also found that Bacteroides, when incubated with an Escherichia coli strain in normal serum, could compete for opsonins and thereby reduce phagocytosis ofE. coli. It is possible that competition for opsonins among bacterial species contributes to the synergistic role these organisms share in mixed floral infections. An effective osponic source including complement or immunoglobulin is required to prepare bacteria for maximal phagocytosis by polymorphonuclear leukocytes (PMNL). Anderson et al.(1) have shown that encapsulated strains of Haemophilus influenzae are opsonized in immune serum by the classical complement pathway, whereas Quinn and co-workers (26) found that these same strains were opsonized by the alternative complement pathway in nonimmune serum. Other investigators (31, 34) have shown that several bacterial species can be opsonized by the complement system in the relative absence of immunoglobulin. In contrast to unencapsulated organisms, encapsulated bacteria appear to require specific antibodies for optimal opsonization (3, 24).Casciato et al. (4) were the first to study opsonization and phagocytosis of Bacteroides species. They demonstrated that B. thetaiotaomicron could be opsonized only by heat-la-bile serum opsonic factors and that phagocytosis by PMNL occurred equally well under anaerobic and aerobic conditions. Recently, Bjornson and Bjornson (2) studied two species of Bacteroides and concluded that optimal opsonization required bot...