Molecular relationship of public H 2 antigens 1, 5, 6, 8, 11, 13, 25, and 28 to private antigens controlled by K and D regions was studied using the technique of antibody induced resistance to complement mediated cytotoxicity. The results indicate physical association in the cell membrane between H 2 antigens 1 and 23 of H 2(a), 8 and 31 of H 2(d), 11 and 17 of H 2(q), 13 and 30 of H 2(q), 25 and 23 of H 2(k), and 28 and 31 of H 2(g). These results are in agreement with genetic mapping placing the determinants of antigens H 2.8, 11, and 25 in the K region, the determinant of antigen H 2.13 in the D region, and the determinants of antigens H 2.1 and 28 in either the K or the D region. In contrast to genetic mapping placing the determinant for antigen H 2.6 in the D region, it was found that in the H 2(b) haplotype the antigen is associated with K region antigen H 2.33. Antigen H 2.5 in the H 2(k) was shown to be associated with both H 2.33 and H 2.32, and this result was interpreted as evidence for 2 homologous H 2.5 sites controlled by opposite ends of the H 2 complex. Although the data do not prove that public antigens are carried by the same molecules as private ones, they demonstrate a close physical association in the membrane between the two groups of antigens. The data provide strong experimental evidence that there are two groups of loci, K and D, coding for the first 33 classical H 2 antigens (with the exception of antigen H 2.7), and thus support the two locus model. The data also support the duplication model of H 2 by demonstrating two homologous H 2.5 sites associated with K and D molecules.