Mice were immunized with preformed complexes of dinitrophenylated haemocyanin (DNP-KLH) and anti-DNP or anti-KLH antibodies, and subsequently assayed for the generation of B memory (BM) cells. Complexes formed at equivalence or in slight antigen excess were far more effective than antigen alone in generating memory: as little as 100 ng DNP-KLH-anti-DNP produced substantial B cell priming. Anti-carrier and anti-hapten antibodies were equally effective. Complexes generated memory more rapidly than antigen alone, and the adjuvant effect was not simply due to aggregation of the antigen. Optimal priming by complexes required the integrity of the Fc portion of the antibody: F(ab')2 antibody fragments were less effective. The capacity of complexes to prime BM cells was abrogated by depriving mice of C3. C3 was also required for localization of complexes within splenic lymphoid follicles; complexes made with F(ab')2 localized in follicles, but less efficiently than those made with intact antibody. These results extend earlier findings (Klaus & Humphrey, 1977) and strongly suggest that the generation of BM cells involves the C3-dependent localization of antigen-antibody complexes within lymphoid follicles and strengthen the concept that germinal centres are the birthplace of BM cells.