The role of the B cell antigen receptor in the induction of somatic hypermutation is presently unclear. We established stable transfectants of the human BL2 cell line expressing hen-egg lysozyme-specific IgM or IgA and compared their ability to induce somatic hypermutation of the endogenous rearranged heavy-chain gene. We found that IgM and IgA were both able to induce somatic hypermutation in an antigen dose-independent manner. The mutations displayed most of the characteristics of somatic hypermutation in vivo. Notably, some replacements introduced stop codons in the coding region. Our data suggest that class-switched memory B cells may undergo somatic hypermutation. They also suggest that the transmembrane/cytoplamic domains of the class-switched isotypes modulatethe signaling and down-modulation activities of the BCR in an antigen dose-dependent manner.