The tumor suppressor gene RB1 is known to be located on chromosome band 13q14. We investigated the involvement of the RB1 gene in a case of refractory anemia with del(13)(q12q14) by florescence in situ hybridization (FISH) analysis using the RB1 locus (13q14) DNA probe. Bone marrow cells derived from this patient exhibited a single signal of the RB1 gene in 58 of 100 bone marrow cells, as determined by interphase FISH analysis. Hematopoietic colony-forming assays showed that the absolute number of erythroid, myeloid, and mixed colonies was comparable to that of normal subjects. FISH analysis of selected colonies revealed that only a single signal for the RB1 gene was detected in five of five granulocyte macrophage-colony-forming units (CFUs), four of five erythroid burst-forming units, and two of four mixed CFUs (total 11/14: 78.6%). Thus, the majority of hematopoietic progenitor cells lacked one allele of the RB1 gene, suggesting that in this particular case the RB1 gene played an important role in abnormal hematopoiesis.