The ADAMs (a disintegrin and metalloprotease) family of proteins is involved in a variety of cellular interactions, including cell adhesion and ecto-domain shedding. Here we show that ADAM 12 binds to cell surface syndecans. Three forms of recombinant ADAM 12 were used in these experiments: the cysteine-rich domain made in Escherichia coli (rADAM 12-cys), the disintegrin-like and cysteine-rich domain made in insect cells (rADAM 12-DC), and full-length human ADAM 12-S tagged with green fluorescent protein made in mammalian cells (rADAM 12-GFP). Mesenchymal cells specifically and in a dose- dependent manner attach to ADAM 12 via members of the syndecan family. After binding to syndecans, mesenchymal cells spread and form focal adhesions-and actin stress fibers. Integrin β1 was responsible for cell spreading because function-blocking monoclonal antibodies completely inhibited-cell spreading, and chondroblasts lacking β1 integrin attached but did not spread. These data suggest that mesenchymal cells use syndecans as the initial receptor for the ADAM 12 cysteine-rich domain-mediated cell adhesion, and then the β1 integrin to induce cell spreading. Interestingly, carcinoma cells attached but did not spread on ADAM 12. However, spreading could be efficiently induced by the addition of either 1 mM Mn2+ or the β1 integrin-activating monoclonal antibody 12G10, suggesting that in these carcinoma cells, the ADAM 12-syndecan complex fails to modulate the function of β1 integrin.