Circulating neutrophil (PMN) levels can increase in rats bearing subcutaneously growing clones of the 13762NF mammary adenocarcinoma and the level of increase correlates with the metastatic potential of the clone. In rats with poorly metastatic MTC tumors, numbers of circulating PMN did not rise, whereas PMN levels rose 50-fold in rats bearing highly metastatic MTLn3, 12-fold in rats with weakly metastatic MTLn2, and 14-fold in those with moderately metastatic MTF7 tumors. Neutrophilia was caused partly by tumor size, but metastatic potential was a stronger determinant, suggesting that PMNs may play a role in the metastatic process. To determine whether circulating PMNs indeed contribute to cellular metastatic potential, we examined effects of PMN on various aspects of the metastatic process. Experimental metastasis assays involving i.v. co-injections of PMNs yielded a dose-dependent increase in extrapulmonary metastases for MTLn3, but no change in lung colonization potential for any of the clones examined. The change in the metastatic profile was not due to any modification in in vivo distribution of i.v. injected tumor cells or in adhesion to endothelial monolayers in vitro. PMNs also had no effect on in vitro DNA, RNA or protein synthesis and were not cytolytic (E:T 100:1). However, PMNs collected from high-passage MTLn3 tumor-bearing rats had a 50% increase in heparanase and type-IV collagenolytic activity as compared to unstimulated PMNs isolated from normal rats. These results indicate that polymorphonuclear cells may contribute to the metastatic potential of highly metastatic clones from the 13762NF mammary adenocarcinoma cells by assisting in the degradation of basement membrane during extravasation.