We have investigated the role of the human alpha(5) beta(1) fibronectin receptor integrin in experimental metastasis. Treatment of human MDA-MB-231 breast carcinoma cells with monoclonal antibodies specific for alpha(5) or beta(1) integrin subunits prior to injection into the tail veins of 7 to 9 week old athymic nude mice significantly decreased the median number of lung colonies that were formed. In contrast, treatment of the cells with monoclonal antibodies specific for the alpha(2) subunit had no significant effect. In vitro, the anti-alpha(5) and the anti-beta(1) monoclonal antibodies both strongly inhibited breast carcinoma cell adhesion to fibronectin, while only the anti-beta(1) monoclonal antibody inhibited adhesion to laminin. In a Boyden chamber invasion assay, the anti-beta(1) antibody almost completely inhibited invasion of the breast carcinoma cells through an artificial Matrigel basement membrane. The anti-alpha(5) monoclonal antibody inhibited in vitro invasion approximately 30%, only if fibroblast conditioned medium was present as a chemoattractant. Cell migration on fibronectin could be inhibited by both the anti-alpha(5) and the anti-beta(1) monoclonal antibody. These results indicate that the alpha(5) beta(1) integrin fibronectin receptor on MDA-MB-231 human breast carcinoma cells plays an important role in experimental hematogenous metastasis and may function in this process by a combination of mechanisms, including tumor cell attachment to fibronectin and fibronectin-directed extravasation of tumor cells into the target organ.