We report the phenotypic effects of transfection of human nm23-H1 cDNA into the human MDA-MB-435 breast carcinoma cell line. Upon mammary fat pad or subcutaneous injection into nude mice, both the nm23-H1 and control transfected lines produced primary tumors; however, the nm23-H1-transfected lines produced metastases in significantly fewer mice than did control transfected lines. Reductions in tumor metastatic potential in vivo were accompanied by decreased colonization in soft agar and an altered colonization response to transforming growth factor beta in vitro. Total nucleoside diphosphate kinase activity, an enzymatic activity possessed by the Nm23 family, was not directly correlated with Nm23-H1 expression levels or suppression of metastatic potential in all cases examined. The data establish that nm23-H1 has functional suppressive effects on the tumor metastatic potential of a human breast carcinoma cell line, and suggest that it may regulate signal responsiveness in the colonization response.