Type 1 insulin-like growth factor receptor (IGF-1R) expression is decreased in prostate cancer compared to that in noncancerous prostate epithelium. We have demonstrated that as the simian virus 40 T antigen (SV40T) immortalized human prostate epithelial cell line, P69SV40T, undergoes transformation from a poorly tumorigenic to a malignant phenotype, the M12 subline, there is a significant decrease in IGF-1R expression. In the present study, we examine the effects of reexpression of the IGF-1R on the malignant phenotype of M12 cells. The IGF-1R was reexpressed in M12 cells using a retroviral vector containing a 7-kilobase coding sequence for the IGF-1R, LISN, to create several clones of the M12-LISN cell line. As a control, M12 cells were also infected with a retroviral vector (LNL6) without the 7-kilobase IGF-1R insert (M12-LNL6 clones). Functional assays were performed with two separate clones each of M12-LNL6 and M12-LISN cells. Each clone of M12-LISN cells regained the proliferative response to IGF that was lost in the transition from P69SV40T cells to M12 cells. In addition, M12-LISN clones had a significantly decreased growth rate compared to the M12-LNL6 cells when injected s.c. in athymic/nude mice (P < 0.001). Tumorigenicity, as assessed by anchorage-independent growth of colonies in soft agar, was also decreased by 75% in the M12-LISN clones compared to that in the M12-LNL6 control cells. These data demonstrate that reexpression of the IGF-1R in a malignant human prostate epithelial cell line results in decreased tumor growth and decreased anchorage-independent colony formation independent of an increased proliferative response to IGF. Reexpression of the IGF-1R may be associated with reacquisition of the regulation of cellular proliferative and differentiation functions mediated by the IGF-1R that are lost as prostate epithelial cells undergo conversion to a malignant phenotype.