There is disclosed movel mutant IGFBP-3 polypeptides and fragments thereof that have either no binding, or diminished binding to IGFs, yet retain their ability to bind to the human IGFBP-3 receptor ("P4.33"). The present invention provides novel mutant IGFBP-3 nucleic acid sequences, and expression systems. Additional embodiments provide for screening assays for identifying IGFBP-3 receptor antagonists or agonists, methods for modulating IGF-independent IGFBP-3 responses of cells expressing IGFBP-3 receptors, A methods for inducing or potentiating apoptosis of cells expressing IGFBP-3 receptors, methods for treating solid tumors having cells expressing IGFBP-3 receptors, and compositions comprising polypeptides having either no binding, or diminished binding to IGFs, yet retain their ability to bind to the IGFBP-3 receptor.