In addition to their role in the immune response, MHC class II antigens may be considered as differentiation markers on hemopoietic cells. To study expression of class II genes at the mRNA level in leukemias representing various stages of lymphoid and myeloid differentiation, we constructed chain- and locus-specific HLA class II DNA probes. As the genes encoding the DR, DQ, and DP beta-chains display a strong sequence homology in the second extracellular and transmembrane domains, we used probes derived from the less conserved 3' untranslated regions. For the more divergent alpha-chain genes, DNA fragments derived from the coding portion were obtained from cDNA clones. All probes were designed to minimize background due to AT- or GC-rich segments and subcloned into pUC plasmids. Their lack of cross-hybridization was demonstrated in Southern blot experiments under moderately stringent conditions. Northern blot analysis of RNA from 15 patients with acute lymphoblastic and myeloblastic leukemias, chronic lymphoid and hairy cell leukemias showed that most patients expressed variable amounts of class II transcripts, some lacked all class II mRNA, and only two patients had a dissociated expression of class II genes, with lack of DQ and presence of DR and DP mRNA. This study reveals a vast heterogeneity of MHC class II gene expression in leukemias, as previously demonstrated at the protein level. The availability of these highly specific DNA probes should prove useful in extensive studies directed at better defining HLA class II gene expression during hemopoietic differentiation in physiologic and pathologic states.