Solid tumors are relatively resistant to growth inhibition by interferons (IFNs). To enhance sensitivity, we assessed combinations of IFNs with tamoxifen in estrogen receptor-positive (ER-positive) and ER-negative human tumor xenografts. In nude mice, the growth of MCF-7 human breast tumors (ER-positive) and NIH-OVCAR-3 ovarian tumors (functionally ER-negative) was suppressed completely when tamoxifen and IFN-alpha or IFN-beta was started 2 days after tumor inoculation. Established, 6-week-old MCF-7 and NIH-OVCAR-3 tumors regressed when treated with the combination of IFN-beta and tamoxifen but not with single-agent therapy. Treatment with the combination also resulted in an augmented antitumor response in vivo in an ER-negative breast tumor (MDA-MB-231), a colon carcinoma (HT-29), and a melanoma (SK-MEL-1). Antiproliferative studies in vitro suggested that growth of both MCF-7 and NIH-OVCAR-3 cells was inhibited to a greater degree by combination treatment with human IFN-alpha and tamoxifen or IFN-beta and tamoxifen compared with single agents. Median effect analysis defined synergy. Four ER-negative carcinomas (MDA-MB-231, MDA-MB-468, BT-20, and HT-29) also exhibited synergistic growth inhibition in response to the drug combination. The response of these four cell lines was particularly striking. Tamoxifen as a single agent had little effect (up to 2.0 microM) but caused enhanced antiproliferative activity when added to IFN-beta. Sequential treatment of MCF-7 cells in vitro with tamoxifen followed by IFN-beta was more effective at inhibiting growth than treatment with IFN-beta followed by tamoxifen, suggesting that tamoxifen modulated the anticellular response to IFN-beta rather than the converse. Similar results were obtained with IFN-alpha. Cell cycle analysis indicated that 7 days of exposure to the combination resulted in MCF-7 cell fragmentation and death. Together with our recent studies demonstrating enhancement of IFN-stimulated gene expression (ISG) by tamoxifen pretreatment in IFN-resistant cells, these data suggest that combination treatment with tamoxifen and IFNs may increase ISG expression in IFN-resistant tumors, leading to augmented antitumor effects. These effects appear to be independent of ER expression.