Hybridoma cell lines secreting antibodies to mouse H-2 or Ia antigens have been generated by fusing mouse immune lymphocytes with appropriate myeloma lines. Among the 11 established clones reported here, nine produce anti-H-2 antibodies and two produce anti-Ia antibodies. The specificities and cross-reactions of these monoclonal antibodies have been studied in detail. One hybridoma antibody reacted only to Kk antigens without any detectable cross-reactions, thus suggesting reaction to a private specificity of the Kk molecule. All other anti-H-2 hybridoma antibodies appeared to detect public specificities as defined either by reactions with products of more than one H-2 locus or with different alleles at one or more loci. The two anti-Ia antibodies both reacted with I-E/C products, but exhibited different cross-reactivity patterns. Strain distribution analyses so far indicate that the public specificities detected by these monoclonal antibodies are considerably different from those that had been established by traditional serology. Since public specificities defined by the hybridoma antibodies must by definition represent cross-reactions, these findings may have important implications relating to the structure and evolution of MHC gene products.