Fig. 1. Growth factor pre-treatment increases cell surface expression of `activated' <sub>v</sub>ß<sub>3</sub>. (A) Unstimulated human osteoclast-like cells were stained with the anti-ß<sub>3</sub> mAb AP5 in the presence of 1.3 mM calcium (AP5+Ca) or in calcium-free buffer (AP5-Ca). In the presence of divalent cations, AP5 recognizes the fraction of the <sub>v</sub>ß<sub>3</sub> integrin in the `activated' conformation. Incubation of cells with AP5 in calcium-free buffer shifts all the receptor into the `activated' conformation. (B,C) Pre-incubation with HGF (B) or with M-CSF (C) increases the binding of AP5 in 1.3 mM calcium buffer, shifting most of the integrin into the `activated' conformation. Mean fluorescence intensity is indicated on the abscissa; the number of fluorescence-positive cells is depicted on the ordinate.