The use of primary, human, ecto- and endocervical epithelial cell cultures has increased our understanding of the pathogenesis of gonococcal infection in women. Primary cervical epithelial cells express complement (C') receptor type 3 (CR3) and C' proteins required for alternative pathway (AP) activity. Gonococcus -induced membrane ruffling and cellular invasion of primary cervical epithelia is mediated by CR3 and requires co-operative CR3 binding by gonococcus-bound iC3b, porin and pilus. We have extended these studies to identify the site of C3 deposition upon gonococci within the cervical microenvironment. By immunoprecipitation and ELISA we demonstrate that covalent and non-covalent associations occurred between gonococcal LOS and C' protein C3. Sialylation or LOS truncation did not alter the gonococcus-CR3 interaction. By Western blot analysis we observed comparable C3 opsonization patterns among a panel of LOS truncation mutants, sialylated wild-type gonococci, or wild-type bacteria that were not sialylated. Quantitative association/invasion assays performed in the presence or absence of LOS competimers support C3b deposition on the lipid A core structure. Our findings demonstrate a role for lipid A as a C3 acceptor site and suggest that multiple factors govern C3b deposition and its subsequent conversion to iC3b on the surface of the gonococcus within the cervical microenvironment.