The effect of estradiol (E2) and tamoxifen (TX) on cytotoxic T-lymphocyte (CTL)-mediated target-cell lysis was studied. CTL was generated in mixed cultures of rat spleen cells, using female Fischer 344 rat cells as responders and female Wistar rat cells or Nb2 rat lymphoma cells as stimulators. Concanavalin-A-stimulated Wistar lymphoblasts or Nb2 cells were used as targets. CTL harvested on day 5 exerted 16-25% cytotoxicity when used at 1:12-1:50 target:effector cell ratios. Day-6 CTL had no cytotoxic activity. Treatment of target cells with either TX or E2, or both, at 1-microM concentrations for 4 hr prior to cytotoxicity testing raised the target-cell killing to 100%. Highly significant enhancement of cytotoxicity was also observed when the drugs were used at 100-, 10-, or 1-nM concentrations. Treatment of effector cells under similar conditions led to inhibition of cytotoxicity at 1-microM concentration, some enhancement at 100 nM and no effect at 10 and 1 nM. When treated target and effector cells were combined, the amplification of target-cell lysis was similar in magnitude to that seen in tests with treated targets only. Drug treatment of target cells had no influence on their resistance to lysis in hypotonic solutions or on total and spontaneous 51Cr release. The inhibition of DNA and protein synthesis in target cells interfered with both basal cytolysis and drug-induced enhancement. The release of the nuclear label, [3H]-thymidine, from Nb2 targets by CTL, was also enhanced by target treatment with TX. These results illustrate that CTL-mediated cytotoxicity is amplified by physiological concentrations (1 and 10 nM) of E2 and equimolar concentrations of TX.