Based on the observation that chromosome 1q deletions are not infrequent in late-stage human breast carcinomas, we tested whether the recently discovered human melanoma metastasis suppressor gene, KiSS-1, which maps to chromosome Iq32-q41, could suppress metastasis of the human breast carcinoma cell line MDA-MB-435. Parental, vector-only transfectants, and KiSS-1 transfectant clones were injected into the mammary fat pads of athymic nude mice and assessed for tumor growth and spontaneous metastasis in regional lymph nodes and lungs. Expression of KiSS-1 reduced metastatic potential by 95% compared to control cells but did not suppress tumorigenicity. Metastasis suppression correlated with a decreased clonogenicity in soft (0.3%) and hard (11.9%) agar. Although the overall rate of cell adhesion to extracellular matrix components was unaffected, KiSS-1 transfectants spread on immobilized type-IV collagen more rapidly than did control populations. Invasion and motility were unaffected by KiSS-1. Based on the predicted structure of the KiSS-1 protein, our results imply a mechanism whereby KiSS-1 regulates events downstream of cell-matrix adhesion, perhaps involving cytoskeletal reorganization. In addition to its already described rule in melanoma, our results show that KiSS-1 also functions as a metastasis suppressor gene in at least some human breast cancers.