The protooncogene Pim-l encodes serine/threonine protein kinases that are involved in cytokine-mediated cell proliferation and in lymphoma-and leukemogenesis. It is largely unknown how Pim-1 executes its biological effects. Here we show that Pim-1 physically interacts with heat shock protein 90 alpha and beta (Hsp90 alpha and beta). The Hsp90-specific inhibitor geldanamycin (GA) induced a rapid degradation of Pim-1 and reduced its kinase activity. The expression of HspS90 alpha was regulated by a signal from the cytokine receptor gp130, as is Pim-1's expression. These results indicate that Hsp90 is coordinately regulated with Pim-1 and is involved in the stabilization and function of Pim-l.