Neisseria meningitidis (meningococcus) is an important cause of meningitis and sepsis. Currently, there is no effective vaccine against serogroup B meningococcal infection. Host defense against neisseriae requires the complement system (C) as indicated by the fact that individuals deficient in properdin or late C components (C6-9) have an increased susceptibility to recurrent neisserial infections. Because the classical pathway (CP) is required to initiate efficient complement activation on neisseriae, meningococci should be able to evade it to cause disease. To test this hypothesis, we studied the interactions of meningococci with the major CP inhibitor C4b-binding protein (C4bp). We tested C4bp binding to wild-type group B meningococcus strain (H44/76) and to 11 isogenic mutants thereof that differed in capsule expression, lipo-oligosaccharide sialylation, and/or expression of either porin (Por) A or PorB3. All strains expressing PorA bound radiolabeled C4bp, whereas the strains lacking PorA bound significantly less C4bp. Increased binding was observed under hypotonic conditions. Deleting PorB3 did not influence C4bp binding, but the presence of polysialic acid capsule reduced C4bp binding by 50\%. Bound C4bp remained functionally active in that it promoted the inactivation of C4b by factor I. PorA-expressing strains were also more resistant to C lysis than PorA-negative strains in a serum bactericidal assay. Binding of C4bp thus helps Neisseria meningitidis to escape CP complement activation.