We have studied the retinoblastoma (RB-1) susceptibility gene status and pRB expression in 22 human myeloma cell lines (HMCL) and in 10 patients with advanced multiple myeloma (MM). Deletions of the RB-1 gene were observed in 81% (17/21) of the informative HMCL, regardless of their paracrine or autocrine interleukin-6 (IL-6) status. Among the deleted HMCL, only one (U266) had a biallelic deletion and lacked pRB expression. Monoallelic deletions had no consequence on the RB-1 gene activation and pRB expression. One patient of 10 presented the same biallclic deletion as U266 and six of 10 had monoallelic deletions. We conclude that monoallelic deletions of the RB-1 gene are frequent in HMCL and MM patients but have no consequence on gene activation and pRB expression.