As part of our continuing effort to produce a library of hybridoma antibodies specific for the products of the mouse major histocompatibility complex (MHC), nine antibodies reacting with antigens of the H-2 d haplotype have been produced by cell fusion between immune spleen cells and the SP2/0. Ag. 14 cell, of H-2 d origin. Serological characterization revealed that seven antibodies reacted with H-2 antigens and two with Ia antigens. Of the anti-H-2 antibodies, four detected private specificities of H-2K d or H-2D d antigens and three detected public specificities of H-2 d and other haplotypes. One of the anti-la antibodies detected a private Ia specificity corresponding to Ia. 23 and the other detected a previously undescribed public specificity.