Circulating polymorphonuclear cell (PMN) levels rise in proportion to the metastatic potential of the tumor in 13762NF mammary adenocarcinoma tumor-bearing rats. These tumor-elicited PMNs (tcPMNs) secrete high levels of the basement-membrane-degrading enzymes, type IV collagenase and heparanase, suggesting that metastatic tumor cells stimulate neutrophilia so that the tcPMNs might assist tumor cell extravasation during metastasis. To test this hypothesis, purified proteose peptone-elicited PMNs from peritoneal exudate, circulating normal PMNs, and tcPMNs were evaluated for their effects on in vitro invasive and in vivo metastatic potentials of syngeneic 13762NF mammary adenocarcinoma tumor cells. tcPMNs caused a dose-dependent increase in invasion through a reconstituted basement membrane barrier in an in vitro invasion assay. At PMN:tumor cell ratios of 30:1, invasion potential significantly (P < 0.05) rose to 26-fold, 40-fold, and 37-fold for poorly metastatic MTLn2 cells, highly metastatic MTLn3 cells, and moderately metastatic MTF7 cells, respectively. In contrast, purified proteose peptone-elicited PMNs and circulating normal PMNs did not significantly alter invasive potential. Intravenous coinjections of purified proteose peptone-elicited PMNs did not change the number of experimental lung metastases, but tcPMNs at ratios to 50:1 significantly raised the mean number of metastases 23-fold for MTLn2, 3- to 4-fold for MTLn3, and 1.6-1.8-fold for MTF7. These results demonstrate that tcPMNs contribute to the metastatic propensity of mammary adenocarcinoma clones by increasing efficiency of invasion through basement membrane.