Fifty-three serum-resistant strains of E. coli which were all able to grow in at least 50% normal human serum (NHS) were tested in respect to binding and consumption of C3b, factor H, C5, and C6 after incubation in pooled NHS. The results of immunofluorescence tests, hemolytic assays, and binding studies using radiolabeled components were comparable. The different binding patterns allowed us to divide the strains into three different groups. The main features of group I were the attachment of C3, C5, and C6 to the bacterial cells as well as consumption of C3 and C5, whereas factor H did not bind at all or only in small amounts. In addition, released MAC was detectable in the supernatant of reaction mixtures containing bacteria of a group I strain and NHS. In group II factor H was easily bound to the bacteria, but no C3, C5, and C6 binding or C5 consumption was detectable. In addition, strains of group III bound C3 and factor H and some strains also bound and consumed C5. Because of the inhomogeneity of group III, this investigation was restricted to a comparison of groups I and II. From the results presented in this study we conclude that group I bacteria activate the whole complement cascade, whereas with bacteria of group II, complement activation is interrupted at the C3 level. These findings therefore indicate a second, alternative mechanism of serum resistance in E. coli.