Human myeloma cells were purified from bone marrow aspirates from four patients having advanced myeloma, including one with common acute lymphoblastic leukemia antigen-positive myeloma. All of these myelomas had marked bone lytic lesions. From the culture supernatants of these purified myeloma cells, bone-resorbing activities were significantly revealed by ⁴⁵Ca-release bone resorption assay, and IL-1 activities were also detected by IL-1 bioas-say (mouse thymocyte comitogenic assay). Sandwich enzyme immunoassay for IL-1α or IL-1β revealed that IL-1β was responsible for IL-1 activity of these culture supernatants. Furthermore, the bone resorbing activities of these culture supernatants were completely neutralized by pre-treatment of anti-IL-1β, but not anti-IL-1α antibody. By Northern blot analysis, IL-1β mRNA was identified from these myeloma cells. Therefore, it is concluded that myeloma cells produce IL-1β, which acts as bone-resorbing activity in multiple myeloma.