The major cellular and molecular factors that lead to the induction of somatic mutations in human B cells remain unclear. This study describes an approach which allowed us to single-cell culture antigen-specific B cells after in vitro immunization. Using single strand conformational polymorphism analysis on the descendant cultures we were able to demonstrate that the Ig gene of B cells was induced to mutate. Three clones were isolated from a single cell culture of naive B cells immunized against the semi-conserved region of V3 of HIV-1, which contained two point mutations at the CDR2 region leading to amino acid codon change. Two clones were also obtained from a single cell culture of memory B cells immunized against the recall antigen tetanus toxoid. These clones showed one point mutation at the CDR1-FR2 border region leading to amino acid codon change. Fab constructs from the mutated culture of naive B cells immunized against V3 were cloned to a procaryotic expression vector. The expression products of two clones showed anti-V3 specificity in ELISA using three different conjugates. Our results suggest that somatic mutations can be induced in human peripheral B cells through specific interaction with antigen and antigen-activated T cells.