We develop our previous observation that a subpopulation of circulating memory IgM(+)IgD(+)CD27(+)B cells belongs to a separate pathway of differentiation in humans. This subpopulation, which represents 5-25% of peripheral B cells, is also present in spleen in the same proportion and displays a marginal-zone-like B cell phenotype. In addition, we describe a short-time in vitro induction model for somatic hypermutation by using the BL2 Burkitt's lymphoma cell line stimulated by a combination of antibodies directed against different surface receptors. This short-time assay allows us to show that mutations are stably introduced in one DNA strand of the BL2 VH gene in the G1 phase of the cell cycle.