Hybridoma-derived monoclonal anti-p-azobenzene-arsonate (ABA) antibodies were obtained from fusions of ABA-KLH primed A/J spleen cells with three different myeloma cell lines. Of the 156 antibody secreting hybridomas 24% carried the cross-reactive idiotype (CRI), which is known to be shared by 20-70% of anti-ABA serum antibodies in A/J mice. The isotypes, SDS-PAGE patterns and the partial amino acid sequences of the V-regions of one CRI negative and six CRI positive hybridoma proteins were determined. These antibodies were IgGl, kappa and IgG2b, kappa. Some idiotype carrying monoclonal antibodies appeared to be serologically identical. Although the partial VH amino acid sequences of these monoclonal antibodies showed great homology with each other and with serum antibody, several sequence variations in framework residues as well as in the first and second complementarily determining regions (CDRs) were found. The cross-reactive idiotype of the anti-ABA antibodies, therefore, exhibits structural microheterogeneity, i.e. it consists of a family of non-identical but closely related molecules as previously reported (Alkan et al., 1980; Estess et al., 1980; Marshak-Rothstein et al. 1980). Here the N-terminal sequence of the VH regions from 14 CRI+ and 8 CRI- antibodies as well as the VL regions from 11 CRI+ and 8 CRI- monoclonal antibodies are compared. Analysis of the available data demonstrated that there are pairs of hybridoma proteins (both CRI+ and CRI-) which have identical sequences for VH or VL. This suggests that there exist a minimum of 4 germ line genes coding for CRI+ VH, CRI+ VL, CRI- VH and CRI- VL respectively. In addition, CRI+ VL has always been found in association with a CRI+ VH.