We evaluated the usefulness of fluorescence in situ hybridization (FISH) using different-colored commercial RB1 and 13qter DNA probes to identify RB1 deletions in interphase nuclei of bone marrow from 24 patients with agnogenic myeloid metaplasia (AMM), 20 patients with multiple myeloma (MM), 21 patients with other hematologic malignancies, and 25 normal bone marrow transplant (BMT) donors. Based on the 25 normal BMT donors, the upper boundary for the normal percentage of nuclei with one RB1 signal was 6.5%. Based on eight specimens known to have a deletion of 13q14 by cytogenetic studies, the lower limit of abnormal for the percentage of nuclei with one RB1 signal was 12.5%. More than 12.5% of nuclei had a single RB1 signal in 7/24 (29%) patients with AMM and 3/20 (15%) patients with MM. None of the 21 patients with hematologic malignancies other than AMM or MM had more than 12.5% nuclei with loss of RB1. The results of this study suggest that FISH with RB1 probes is useful to detect loss of RB1 in interphase nuclei from patients with hematologic disorders who have chromosome abnormalities involving 13q14. Thus, FISH with probes for RB1 is efficacious to investigate the pathogenesis of RB1 in malignant neoplasms and is a useful adjunct to conventional cytogenetic studies in clinical practice when abnormalities of 13q14 are involved.