In order to test the relative immune competence of a minor VH subgroup (y), the concentrations of rabbit Ig bearing the VH subgroups a and y were determined in experimentally immunized rabbits. Three groups of three rabbits each were immunized with either Micrococcus luteus, bovine serum albumin or an alloantigenic determinant (b4) of the rabbit kappa light chain. The ratios of VHy to VHa Ig in the pre-immune sera ranged from 1:8 to 1:283. In five of the nine rabbits, the VHy to VHa ratios in affinity purified antibody (Ab) closely reflected the ratios of VHy to VHa in pre-immune serum. Of the remaining four rabbits, three produced disproportionately low amounts of VHy Ab, when compared to pre-immune values, while one rabbit produced virtually no VHy Ab. Regardless of the levels of VHy and VHa Ab detected, the kinetics of Ab production by each subgroup during the course of the immune response were similar to each other. When total serum levels of the subgroups were analysed, all rabbits displayed increases (6-450%) in total serum levels of both VHa Ig and total IgG following immunization, while in eight out of nine rabbits the level of VHy Ig remained unchanged. Our observations suggest that the immune repertoire of the minor VHy subgroup is sufficiently diverse to generate VH regions which react with a variety of antigens, and in five out of nine rabbits, VHy Ab-producing clones compete well with VHa Ab-producing clones. In addition, it appears that clones of the major and minor VH subgroups, once triggered, are under similar regulatory controls with respect to specific Ab, but differ with respect to non-specific Ig production.