Somatic mutations that are acquired by antibody V genes of antigen-stimulated B cells ultimately provide the clonal diversity from which memory B cells are selected during immune responses to T-cell-dependent antigens. Somatic mutations apparently are not acquired when B cells are stimulated by mitogens nor when they participate in immune responses to T-cell-independent antigens. Since the basis of T-cell-dependent humoral immunity is T-cell recognition of processed antigen in the context of class II major histocompatibility glycoproteins (Ia) on the B-cell surface, we sought to determine whether the ligation of Ia on B cells induces somatic mutation. B cells were stimulated in vitro by a procedure in which their proliferation was dependent upon ligation of surface Ia with antibody. Sequences of hybridoma V genes derived from these B cells revealed no somatic mutations despite prolonged stimulation in vitro and the induction of immunoglobulin secretion and switching to isotypes characteristic of T cell-dependent humoral immunity. We infer that Ia-mediated signalling and isotype switching are not causally related to somatic mutation. The avenue of differentiation that leads to somatic mutation in memory B cells is apparently separable from that leading to proliferation, immunoglobulin secretion and switching.