Idiotypic determinants characterizing certain antibody specificities have proven valuable structural and genetic markers in studies of antibody diversity and regulation. The major cross-reacting idiotype in strain A/J mice immunized with para-azophenylarsonate is heritable and encoded by germline genes. We have demonstrated (in collaboration with M. Gefter) that hybridoma proteins bearing this predominant idiotype are serologically and structurally microheterogeneous but are all derived from a single VH germline gene. In addition, a set of arsonate-nonbinding hybridoma proteins bearing this same predominant idiotype have been produced by immunization with anti-idiotype. Structural studies have demonstrated that these anti-idiotype antibodies are closely related to the arsonate-binding, idiotype-bearing antibodies and are derived from the same VH and VL germline genes. The loss of antigen binding in these molecules has been correlated with somatic mutation involving either the VH gene and/or JH gene segments. In addition, among arsonate-binding hybridomas it is possible to identify a set that have lost idiotype by virtue of somatic mutation in the VH gene or by utilization of different D-gene segments than are ordinarily utilized. The Fab fragment from an arsonate-binding, idiotype-bearing hybridoma has been crystallized, which makes it likely that the three-dimensional structure responsible for this idiotype will be known. In addition to the predominant idiotype, a second idiotype family (Id36-60) among A/J antiazophenylarsonate antibodies, which are structurally and serologically distinct from the predominant idiotype, have been characterized. The complete variable region protein sequences of Id36-60 hybridomas for both the A/J and BALB/c strains have been determined. The entire Id36-60 family arises by somatic mutation from single germline VH genes in each strain which are closely related. In addition, the complete light chain variable region sequences of hybridomas from the two strains bearing Id36-60 have been determined. These studies, in combination with chain recombination studies, indicate that the protein encoded directly by the germline gene in the BALB/c strain is associated with low affinity for the antigen, indicating that somatic mutation in the system is necessary to enhance arsonate affinity. (AB)