Several rabbit xenoantisera raised against human serum Ia antigens have been shown to detect murine lymphocyte antigens. The murine system detected was shown to be polymorphic and expressed on B cells and not T cells. Serological analysis of recombinant H-2 strains demonstrated that two of the antisera recognized products of the I-Cd subregion, and one antiserum recognized products of the I-Cb subregion, thus clearly establishing the existence of the I-C subregion in these two haplotypes. However, as the I-Cd subregion is classically described by the Ia.6 specificity present on T cells and xenoantisera detect a product on B cells, the indication is that the I-C region, like I-A and I-J, complex and contains genes coding for specificities on T cells or B cells.