The nucleotide sequences of the variable regions of lambda 1 chain bearing anti-NP antibodies from the secondary response of C57BL/6 mice were determined. The data indicate that the V186.2 VH gene which dominates the primary anti-NP response is expressed in nine out of 10 secondary response antibodies and is extensively mutated. In the V lambda 1 regions somatic mutations are less frequent. While point mutations predominate, there is suggestive evidence for two conversion events, one involving a one-codon deletion. Most, but not all, secondary response antibodies have a higher affinity (up to 10-fold) for the hapten than is seen in the primary response. The increase in affinity correlates with 'parallel' mutations in CDRs of H and L chains, likely to play a role in hapten binding. The analysis of VDJH rearrangements demonstrates that the secondary response lambda 1 chain-bearing antibodies are produced by a diverse set of B cell clones, which are only rarely expressed in primary responses. These clones are characterized by N-sequence-mediated heterogeneity in the 3' half of CDR3, where the germ line sequence of the D element DFl16.1 predominates in primary response antibodies. The antibodies analyzed in this and in previous work were isolated from idiotypically suppressed mice in order to evaluate whether, intraclonally, idiotype suppression selects antibody mutants into the memory pool, through suppression of the wild-type. A selection of this type was not detectable. However, idiotype suppression may control the pattern of clonotypes expressed in the primary versus the secondary response.