Human and murine tumor cells contain cell surface receptors for the basement membrane glycoprotein laminin. Since a biologic role for the receptor had not previously been demonstrated, we explored the possibility that the laminin receptor may be involved in hematogenous metastases formation. Preincubation of metastatic murine melanoma cells with syngeneic whole laminin followed by tail vein injection increased tumor cell retention in the lung and strongly stimulated metastases formation. The domain of the laminin molecule responsible for stimulating metastases was identified. Laminin is a cross-shaped molecule with three short arms and one long arm. All arms have globular end regions. Purified protease-derived fragments of laminin were prepared which (a) lacked only the long arm of the molecule (alpha fragment) or, (b) lacked both the long arm and the globular end regions of the short arms (C1 fragment). Both types of fragments contained the laminin receptor binding region. The fragments had opposite effects on metastases. The alpha fragment stimulated metastases formation to the same extent as whole laminin. In contrast, the C1 fragment greatly reduced or abolished metastases formation in a dose-dependent manner. The C1 fragment also inhibited tumor cell attachment to whole amnion basement membrane in vitro. We conclude that intact globular end regions on the short arms (but not the long arm) of the cell surface receptor-bound laminin molecule are necessary for stimulating metastases by the intravenous route.