Since the discovery of the H-2Ld molecule (Lemonnier et al. 1975) we have demonstrated that several K and D region alleles produce more than one type of H-2 molecules. Two of four different molecules were distinguished in the products of different alleles. Some of these molecules are products of different genes (H-2D, H-2L), in other instances the evidence for distinct genes is not available. Some of the different molecules produced by the same region might be modified products of the same gene. In the instances where no information implicating different genes is available, we use a neutral terminology which does not presume a genetic difference: H-2K1d and H-2K2d, H-2D1k, H-2D2k, H-2D1dx, H-2D2dx, H-2L1d, H-2L2d, etc. Immunoprecipitation experiments with some anti-H-2L and anti-Qa-2 sera revealed proteins with the apparent molecular weight of 41,000. We designate these antigens provisionally Lq and Qx, respectively. The Lq protein is polymorphic and it is at least partly under the control of H-2L-linked genes since it is absent from BALB/c-H-2dm2 cells. Since we have never seen the 41,000 proteins in precipitates of H-2K or H-2D antigens, it appears that whatever the origin of these molecules, they reveal some features common to products of L and Qa region. The basic relationship of H-2 K, D, L antigens is revealed also by the shared antigenic specificities between these H-2 molecules which we demonstrate using anti-H-2.28 sera. In summary, our results show that the class I antigens in each haplotype represent a family of several distinct but antigenically related molecules. The specificities of the H-2.28 family are the strongest allotype common to different H-2 K, D, and L molecules. Recent direct demonstration of several different genes in the Dd region (Steinmetz et al. 1981) provides evidence for the genetic complexity of H-2 genes which may be underlying basis of the molecular heterogeneity of H-2 antigens discussed here.