Tissue infiltrating lymphocytes isolated from the preneoplastic mouse mammary hyperneoplastic alveolar nodule (HAN) tissue line C4 express high levels of natural killer (NK) activity, which gradually wanes as spontaneous tumors develop (W. Z. Wei and G. Heppner. Br. J. Cancer, 55: 589-594, 1987). Experiments were performed to test whether modulation of NK cell activity would be associated with altered progression of HAN to tumor. Administration of polyinosinic-polycytidylic acid, which activates NK activity but does not directly affect mammary epithelial cell growth, to HAN-bearing mice enhanced tumor progression, as measured by a decrease in the latency period and increase in the incidence of mammary adenocarcinomas developing in the HAN implants. Antiasialo GM1, which reduces NK activity, reduced tumor progression. The net effect of indomethacin, which may inhibit mammary epithelial cell growth but enhances NK cell function, was to prolong the latency period of tumor development. However, this effect was reversed by interleukin 2, which activates NK cells. These findings suggest that NK activity may provide positive signals for progression of preneoplastic mammary lesions to frank neoplasia.