Deletion of the human retinoblastoma gene in primary leukemias. As an initial step in evaluating the role of tumor suppressor genes in leukemogenesis, we surveyed primary leukemia cells from 130 patients for possible deletion of the retinoblastoma susceptibility (Rb) gene by Southern blot analysis. Two of them clearly showed homozygous deletion of Rb alleles. The first patient was a pre-B acute lymphoid leukemia (ALL) associated with a cytogenetic translocation: t(14;16)(q24;q22). The deletion was located at the 3' portion of the Rb gene, very close to the site of Rb gene deletion recently identified in an ALL cell line. The absence of Rb110 protein was further confirmed by Western blot analysis. The second patient was a chronic myelomonocytic leukemia (CMMoL), terminated in acute blastic transformation. Deletion of the 5' portion of Rb gene was found in leukemic cells in the chronic stage. The results indicated that inactivation of the Rb gene occurred in certain cases of leukemia. Its significance warrants further study. 