We determined whether the IFN-β gene can be used to suppress angiogenesis, tumor growth, and metastasis of human prostate cancer cells growing in the prostate of nude mice. Highly metastatic PC-3M human prostate cancer cells were engineered to constitutively produce murine IFN-β subsequent to infection with a retroviral vector containing murine IFN-β cDNA. Parental (PC-3M-P), control vector-transduced (PC-3M-Neo), and IFN-β-transduced (PC-3M-IFN-β) cells were injected into the prostate (orthotopic) or subcutis (ectopic) of nude mice. PC-3M-P and PC-3M-Neo cells produced rapidly growing tumors and regional lymph node metastases, whereas PC-3M-IFN-β cells did not. PC-3M-IFN-β cells also suppressed the tumorigenicity of bystander nontransduced prostate cancer cells. PC-3M-IFN-β cells produced small tumors (3–5 mm in diameter) in nude mice treated with anti-asialo G_M1 antibodies and in severe combined immunodeficient/Beige mice. Immunohistochemical staining revealed that PC-3M-IFN-β tumors were homogeneously infiltrated by macrophages, whereas control tumors contained fewer macrophages at their periphery. Most tumor cells in the control tumors were stained positive by an antibody to proliferative cell nuclear antigen; very few were positively stained by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling. In sharp contrast, PC-3M-IFN-β tumors contained fewer proliferative cell nuclear antigen-positive cells and many terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling-positive cells. Staining with antibody against CD31 showed that control tumors contained more blood vessels than PC-3M-IFN-β tumors. PC-3M-IFN-β cells were more sensitive to lysis mediated by natural killer cells in vitro or to cytostasis mediated by macrophages than control transduced cells. Conditioned medium from PC-3M-IFN-β cells augmented splenic cell-mediated cytolysis to control tumor cells, which could be neutralized by antibody against IFN-β. Collectively, the data suggest that the suppression of tumorigenicity and metastasis of PC-3M-IFN-β cells is due to inhibition of angiogenesis and activation of host effector cells.