2MeSADP [β-32P] (6 ci/m mole) was synthesized and purified by HPLC. 2MeSADP aggregated platelets and inhibited the stimulation of adenylate cyclase by PGE1 with activities relative to ADP of 4 and 300 respectively. This selectivity for the cyclase site confirms the conclusion from other evidence that this site differs from the site that mediates shape change and aggregation. [B-32P] 2MeSADP bound rapidly (<20 sec) to platelets in PRP with half saturation at 18 nM and 450-500 binding sites per Platelet Under the same conditions PCE1-stimulated adenylate cyclase activity was 50% inhibited at 7 nM with a Hill coefficient of unity. With platelets in Tyrodes solution the apparent affinity was 2-4 nM. Other nucleotides competed for binding to washed platelets with the following Ki (μM):ADP, 3; ATP, 7;IDP, 150; GDP, 350 and AMP, 790. Binding was abolished by 100 μM p-chloromercuribenzene sulfonate, which blocks the inhibition of adenylate cyclase without affecting shape change. Bound 2MeSADP was completely displaced by excess ADP, with first order rate constant 1.54 mIn. These results, taken together with binding data for dlhydroergocryptine (Newman, et al JC1, 61: 395, 1978) and for prostacyclin (Siegl, et al, JCI, in press), suggest that the human platelet hae ca. 200-500 adenylate cyclase units per cell, each coupled to several specific receptors.