Previous studies have shown that platelet membranes bind radiolabeled ADP and have nucleoside diphosphokinase activity which transforms added ADP to ATP. In order to further characterize these reactions, the ADP-binding and nucleoside diphosphokinase activity of purified platelet membranes were solubilized by freeze-thaw injury followed by extraction with isotonic buffered saline. Up to 80% of membrane ADP-binding activity was solubilized along with 20% of the total membrane protein, a 4-fold purification. A Millipore filter binding assay was developed to detect the soluble binding protein using [3H]ADP as radioligand. Binding of [3H]ADP was rapid, reversible, saturable, and was destroyed by heat, trypsin digestion, and 1 mM N-ethylmaleimide. By Scatchard analysis, there was a single class of binding sites with a Kd of 3.8 x 10(-7) M. Unlabeled nucleotides competed with [3H]ADP with the following potency series: ATP = ADP greater than AMP greater than adenosine. The solubilized nucleoside diphosphokinase activity could be separated from ADP-binding activity by ultracentrifugation on 5 to 20% sucrose density gradients containing 0.6 M KCl suggesting that the activities reside on separate molecules. Hydrodynamic parameters were calculated for the binding protein by gel filtration and ultracentrifugation. The s20,w was 4.1, Stoke's radius 35 x 10(-8)cm, axial ratio (f/fo) 1.09, and the Mr = 61,000. The studies suggest that this platelet ADP-binding protein may act as the receptor for initiating ADP-induced aggregation and release.