Studies on the development of the immune response suggest that the repertoire of expressed antibody specificities is strongly influenced by antigen (reviewed in ref. 1). One way in which this influence is manifested is by a progressive increase in the affinity of antibody for antigen with time after immunization. This phenomenon, termed the 'maturation' of the immune response, must be due to a change in the structure of the antibody being synthesized. However, the precise nature of the changes involved and the genetic mechanisms used to produce them have not been clearly defined. We have now investigated the maturation of the immune response to the hapten 2-phenyloxazolone by mRNA sequencing of specific hybridomas. We conclude that somatic mutation of germ-line encoded genes plays a major role in the generation of antibodies with increased affinity for oxazolone with time after immunization.