The effect that polymorphonuclear leukocytes (PMN) may have on the metastatic colonization of tumor cells is controversial: some laboratories have reported that PMN can inhibit metastasis whereas others have shown an augmentation effect. We have exploited our finding that a particular fibrosarcoma (BMT-I I) transplanted subcutaneously into syngeneic C57BL16 mice induces a progressive increase in the number of circulating PMN, to re-examine this question. We used such "granulocytosis-positive" mice as recipients of B16 melanoma cells to examine the influence of significant granulocytosis on the level of lung tumor colonies. The number of 816 melanoma lung colonies detected after intravenous 0.v.) injection was significantly higher in BMT-I I tumor-bearing mice with granulocytosis than in control (non-tumor-bearing) mice. Retention of '251UdR-labelled B16 cells 24 hr after the i.v. injection was 3 to 10 times greater in mice with granulocytosis than in controls. Either simultaneous injection, or preinjection of PMN with 816 cells, increased the lung-colonizing capacity of B I6 melanoma cells. These results suggest that abnormally increased numbers of PMN in the peripheral blood, particularly in the lung circulation, can enhance the ability of tumor cells to colonize or metastasize.