Two anti-H-2Ld sera were analyzed, BALB/c-H-2dm2 anti-BALB/cBy and (C3H X BALB/c-H-2dm2)F1 anti-BALB/cHe, the latter also containing anti-Qa antibodies. Their reaction patterns were compared with an anti-Qa serum (C3H X BALB/cBy)F1 anti-BALB/cHe. Four H-2 specificities could be detected by the anti-H-2Ld sera, two already known (H-2.64, H-2.65) and two new specificities (H-2.81, H-2.82). According to their reaction pattern H-2.64, H-2.81, and H-2.82 can be regarded as members of the H-2.28 family of specificities. A quantitative difference in the expression of these H-2 specificities exists in different haplotypes. The cells of the strain against which the sera were made (BALB/cHe and BALB/cBy, respectively) did not give the highest titers with the antisera and had a relatively low absorbing capacity. The H-2dx haplotype carries two new specificities of the H-2.28 family, namely, H-2.81 and H-2.82. Lysostrip tests showed that the antibodies against those specificities cap the H-2.1-positive H-2Ddx molecules, suggesting that these molecules may react with both anti-H-2.1-like and anti-H-2.28-like antibodies. The H-2 specificities detected by the BALB/c-H-2dm2 anti-BALB/cBy serum were detected also in liver, kidney, spleen, heart, and lung tissue. New information on the strain distribution of Qa-2 was obtained from the experiments and a quantitative difference in Qa-2 antigens between H-2 congenic strains was observed as well. The H-2b strains react with these antibodies with higher titers than the strains carrying the H-2d haplotype.