Four MAbs recognizing HLA class II antigens were produced by immunization with human leukemic cells, and were characterized through immunoprecipitation and cell distribution studies. They were tested against a panel of normal and leukemic cells, Epstein-Barr virus (EBV)-transformed homozygous typing cells (HTC)s, deletion mutant cell lines, and 11 other cell lines. The immunochemical studies revealed the presence of two bands of 34 and 28 kD, corresponding to the alpha- and beta-subunits of HLA class II antigens, respectively. The cell distribution studies led to the conclusion that GRB1, GRB2, and GRB3 MAbs recognize the HLA-DR monomorphic antigens, and that the GRB4 MAb seems to recognize basically the DR molecules but shows also a cross-reaction to DP molecules.