The binding of adenosine and adenylnucleotides to the human platelet membrane has been measured and the subsequent effect on the binding of 14C-ADP has been determined. The following conclusions have been drawn: (1) Two reactions with different velocities (t 1/2 = 15–30 sec and t 1/2 = 4–6 min) seem to be involved in the binding of ADP to the platelet membrane. (2) AMP binds to the platelet membrane and partially inhibits binding of ADP in a competitive manner. These two findings suggest that ADP binding sites on the platelet membrane may be of more than one type. (3) At concentrations up to 10-5 M, ATP does not itself bind to the platelet membrane but is rapidly transformed into ADP which, in turn, binds to the membrane and inhibits the binding of 14C-ADP. (4) Adenosine does not significantly interfere with the binding of 14C-ADP and has not been shown to bind to the platelet membr