It is possible to prepare antisera that recognize individual members of a family of hybridoma proteins in which all members of the family express the cross-reactive idiotype (IdCR) characteristic of the A/J anti-Ars response. These hybridoma-specific reagents also recognize a small percentage of the IdCR+ anti-Ars antibodies, which constitute the normal A/J anti-Ars response. Thus, the previously reported sequence heterogeneity of our IdCR+ hybridoma proteins actually represents components of the normal A/J anti-Ars response. Determinants associated with one particular IdCR+ hybridoma protein, 36-71 (Id36-71), were found on approximately 0.01 to 5.0% of the IdCR+ antibodies in all of the A/J hyperimmune sera that were examined. Determinants associated with 2 other IdCR+ hybridoma proteins, 16-46-4-8 (Id16-46) and 44-10 (Id44-10) were also found in almost all A/J immune sera, again at various concentrations. These results indicate that the heritable trait (IdCR) is expressed as a large number of antibody sequences that probably arise from the somatic diversification of a limited number of germ line genes. The pattern of diversification is qualitatively similar among individual mice but varies with regard to the relative proportions of the individual antibody components.