CDC37 was originally identified as a Start gene in budding yeast and has been shown to be required for association of CDC28 with cyclins. The exact functional mechanism by which CDC37 promotes this association, however, remains unknown. CDK4 is a cyclin D-dependent kinase that controls progression through G1 of the mammalian cell cycle. We have detected a specific association of CDK4 with the molecular chaperon HSP90 and a 44-kDa protein that we identify as mammalian CDC37. A physical interaction between CDC37 and CDK4 suggests that CDC37 may regulate the mammalian cell cycle through a direct effect on CDK4. Association of CDK4 with both CDC37 and HSP90 may also imply a mechanistic link between the functions of CDC37 and HSP90.