We have studied the maturation of the immune response by looking at the generation of antibody diversity in germinal centre B cells. Mice were immunized with the antigen 2-phenyloxazolone. Germinal centre B cells, defined by their strong binding to peanut agglutinin (PNA(hi], were sorted from the spleen and fused. Ten days after immunization high numbers of antigen specific hybridoma lines were obtained from the PNA(hi) subset of B cells, suggesting that the small fraction of B cells which are PNA(hi) harbour the antigen activated population. The majority of the day 10 sequences from PNA(hi) cells were shown to be mutated. However, in contrast to results from later stages of the immune response, most of the mutations found were silent. The preferential expansion of B cell clones expressing the mutations characteristic of the mature response was not observed at this stage. Among these hybridoma lines was at least one which, apparently through somatic mutation, had lost the ability to bind antigen. We conclude that in the micro-environment of the germinal centre the B cell repertoire is diversified by hypermutation.