To investigate age-related alterations in human humoral immunity, we analysed Ig heavy chain variable region genes expressed by peripheral B cells from young and aged individuals. Three hundred and twenty-seven cDNA sequences, 163 µ and 164 γ transcripts with V_H5 family genes, were analysed for somatic hypermutation and V_HDJ_H recombinational features. Unmutated and mutated µ transcripts were interpreted as being from naive and memory IgM B cells, respectively. In young and aged individuals, the percentages of naive IgM among total µ transcripts were 39% and 42%, respectively. D and J_H segment usage in naive IgM from aged individuals was similar to that from young individuals. The mutational frequencies of memory IgM were similar in young and aged individuals. γ transcripts, which are regarded as being from memory IgG B cells, showed a significantly higher mutational frequency (7·6%) in aged than in young individuals (5·8%) (P < 0·01). These findings suggest that V_HDJ_H recombinational diversity was preserved, but that the accumulation of somatic mutations in the IgG V_H region was increased in aged humans. The accumulation of somatic mutations in IgG B cells during ageing may imply that an age-related alteration exists in the selection and/or maintenance of peripheral memory B cells.