C3 participates in lymphocyte cooperation leading to antibody formation and in antigen-localization by germinal centers. These functions probably involve the adherence of fixed C3b to complement receptors on lymphocytes and possibly macrophages. During investigation of the role of C3 in antibody formation in human and murine in vitro culture systems, the interaction of purified fluid phase C3b with lymphocyte complement receptors was studied by immunofluorescence. Human peripheral blood and tonsil complement receptor lymphocytes (CRL) were stained by incubation with human or murine C3b followed by specific F(ab)2 anti C3-FITC. Murine CRL failed to stain with these same reagents, despite the fact that murine C3b blocked formation of complement-dependent rosettes. Binding of mouse C3b to murine CRL was directly demonstrated using C3b itself labeled with fluorescein and could be blocked by unlabeled mouse C3b. It affected the same cells as formed C3-dependent rosettes, none of which were T lymphocytes.