Treatment of C57BL/6J mice with poly-I:C or antibodies to asialo-GM1 enhances and depresses respectively natural killer (NK) cell activity while inversely altering lung metastasis, suggesting a critical role for these cells in controlling tumor formation. We assessed the effect of these treatments on antitumor activity mediated by macrophage (M phi) populations likely to be important in lung metastasis. Alveolar and lung interstitial M phi were asialo-GM1 positive (98%) and were sensitive to in vitro treatment with the antibody plus complement; however, treatment of mice with antibodies to asialo-GM1 failed to alter their tumoricidal activity in vitro. Few blood monocytes (4%) or spleen M phi (2%) were asialo-GM1 positive and their antitumor activity was similarly unaffected. In contrast, however, this same in vivo treatment resulted in a 14-fold increase in lung metastasis. Intraperitoneal injection of poly-I:C greatly reduced metastasis formation but also failed to significantly affect in vitro cytolytic activity of the M phi populations. These findings suggest that the major metastasis altering effects of these agents result from modulation of NK rather than M phi function.