BACKGROUND The nonsteroidal antiestrogenic drug, tamoxifen, inhibits the growth of estrogen receptor-positive tumors by interfering with the growth-stimulatory effect of estradiol. However, there is compelling evidence that tamoxifen treatment also is beneficial for patients with estrogen receptor-negative tumors. The hypothesis that tamoxifen is capable of enhancing the immunologic defense of tumor-bearing hosts was been investigated as a possible method for targeting receptor-negative neoplasms. METHODS Natural killer (NK) cells in the spleen of female Fisher and Wistar-Furth rats were used against the YAC-1 murine lymphoma target in 51Cr-release assays. The effect of various concentrations of estradiol and tamoxifen (1 nM-1 microM) and of the metabolic inhibitors actinomycin D and cyclohexamide on target-cell killing was investigated. RESULTS Tamoxifen enhanced and estradiol inhibited killing if applied for the entire 5 hours of the cytotoxic reaction. When applied jointly in this experimental setup, estradiol interfered with the enhancing effect of tamoxifen. Pretreatment of target cells with tamoxifen led to highly significant enhancement of cytotoxicity; estradiol also enhanced target cell killing, but to a lesser extent. After joint treatment, the level of cytotoxicity was comparable with that obtained with tamoxifen alone. Both pharmacologic (100 nM and 1 microM) and physiologic (1 or 10 nM) concentrations of estradiol and equimolar tamoxifen enhanced target cell lysis. However, pharmacological levels of estradiol inhibited effector cells when applied alone or in combination with tamoxifen. Highly significant enhancement of target-cell destruction occurred if both target and effector cells were pretreated with tamoxifen, whereas estradiol treatment of both cell types led to slight enhancement or no effect on cytotoxicity. Treatment of the target cells with actinomycin D or cycloheximide inhibited the lysis of untreated and tamoxifen- or estradiol-exposed cells. Treatment of YAC-1 target cells with tamoxifen or estradiol also enhanced the NK cell-mediated release of the nuclear label, 3H-thymidine, indicating DNA degradation. Similarly treated P815 cells resisted lysis by NK cells, but showed sensitization when the NK cells were stimulated by interleukin-2 for 48 hours before the lytic reaction. Estradiol and tamoxifen changed the kinetics of 3H-thymidine incorporation by YAC-1 cells, but the cells were capable of growing with the highest drug concentrations (1 microM) used in the cytotoxicity experiments. YAC-1 cells have no cytosolic estradiol receptors and are weakly positive for cytosolic progesterone receptors. CONCLUSIONS These experiments indicate that NK cell-mediated target-cell destruction can be enhanced by tamoxifen primarily through sensitizing the target for lysis. Estradiol also sensitizes the target but inhibits the effector cells simultaneously so that little or no change results in cytolysis. Target-cell sensitization is not mediated by classical estrogen receptors and requires the active metabolic participation of the cells treated. A likely mechanism of this phenomenon is priming the target cell for apoptosis.