Infants respond to antigen by making antibody that is generally of low affinity for antigen. Somatic hypermutation of immunoglobulin genes, and selection of cells expressing mutations with improved affinity for antigen, are the molecular and cellular processes underlying the maturation of antibody affinity. We have reported previously that neonates and infants up to 2 months of age, including individuals undergoing strong immunological challenge, show very few mutated V(H)6 sequences, with low mutation frequencies in mutated sequences, and little evidence of selection. We have now examined immunoglobulin genes from healthy infants between 2 and 10 months old for mutation and evidence of selection. In this age group, the proportion of V(H)6 sequences which are mutated and the mutation frequency in mutated sequences increase with age. There is evidence of selection from 6 months old. These results indicate that the process of affinity maturation, which depends on cognate T-B cell interaction and functional germinal centres, is approaching maturity from 6 months old.