Adenosine 5'-diphosphate (ADP) and, uniquely, its C-2 derivatized analogues are able to induce platelet activation. We here report the synthesis of 2-[(3-aminopropyl)thio]-ADP from ADP itself via 1,N6-etheno-ADP. 2-[(3-Aminopropyl)thio]-ADP induced platelet aggregation with a potency about one-seventh that of ADP itself and should prove a useful intermediate in the synthesis of other probes of platelet function.