Vitronectin is a multifunctional adhesive glycoprotein found in the cir culation and different tissues. Compelling experimental evidence, accumu lated by several laboratories during the last few years, suggests that vitronectin provides a unique regulatory link between cell adhesion, hu moral defense mechanisms, and cell invasion. An IX,-glycoprotein was initially enriched from human serum by adsorp tion onto glass beads (Holmes 1967). Although the cell attachment activity of this" serum-spreading factor"(Barnes et al 1980) is similar to the activity of fibronectin, both proteins are biochemically different and