Partially purified mouse interferon type I (Mu IFN-beta), crude mouse interferon type II (Mu IFN-gamma) serum, and the interferon inducer polyinosinic acid-polycytidylic acid (poly I:C) were evaluated for their effects on the growth of spontaneously occurring mammary carcinomas in BALB/c mice. As soon as their tumors had reached a palpable size (diameter: 3-5 mm), the mice received three ip injections of Mu IFN-beta, Mu IFN-gamma, or poly I:C per week for 6 weeks. A significant (fourfold to fivefold) reduction in tumor size was achieved with Mu IFN-beta (at 10(6) U/mouse), Mu IFN-gamma (at 10(3) U/mouse), and poly I:C (at 1 mg/mouse). A similar reduction in tumor size was noted when the mice were treated with cyclophosphamide (2.5 mg/mouse, three ip injections per wk for 6 wk). Treatment with Mu IFN-beta combined with Mu IFN-gamma resulted in a greater tumor growth-inhibitory effect than treatment with either Mu IFN-beta or Mu IFN-gamma alone. In addition to inhibiting the growth of primary tumors, interferon also reduced the incidence of lung metastases. However, complete tumor regression was not observed in any mouse while under interferon therapy.