Three putative HLA-DC-specific monoclonal antibodies, Genox 3.53, BT3/4 and anti-Leu-10, and the HLA-DR-specific antibody, L243, were compared. Their interactions with molecules from homozygous cell lines expressing DR types 1 through 9 were studied. Indirect radioimmunoassays on 29 cell lines demonstrated that Genox 3.53 reactivity correlated with DR1, 2, 6; BT3/4 reactivity correlated with DR 1, 2, 4, 6, 8; and anti-Leu-10 reactivity correlated with DR1,2,4,5,6,8, and 9. In addition, one of six DR3-positive cells and three DR7, DRw10-positive cells reacted with anti-Leu-10 and one of two DR9-positive cells reacted with BT3/4. Binding studies with soluble antigen and competitive radioimmunoassays demonstrated that all three antibodies reacted with the DC1 molecule. Preincubation with BT3/4 blocked anti-Leu-10 binding; Genox 3.53 and L243 did not. Genox 3.53 and L243 were only blocked by themselves. Serial immunoprecipitation showed anti-Leu-10 reacted with non-HLA-DR molecules from cells expressing DR types 1-6, 8 and 9. However, the molecules precipitated by anti-Leu-10 were characteristic class II major histocompatibility complex (MHC) molecules. Their alpha and beta chains were of lower apparent molecular weight than the DR chains in all haplotypes. They also comigrated with the DC1 molecule precipitated by Genox 3.53. Serial immunoprecipitation also showed that anti-Leu-10 removed all Genox 3.53 reactive molecules from cell lysates, but Genox 3.53 removed only a subset of anti-Leu-10 reactive molecules. These studies show Genox 3.53, BT3/4, and anti-Leu-10 react exclusively with class II MHC molecules that are not HLA-DR, and most likely define different polymorphisms of DC molecules, the human equivalent of mouse I-A products.