Aggregation of human blood platelets induced by ADP was strongly inhibited by 2-azidoadenosine, a photolysable analogue of adenosine. The deamination of adenosine by adenosine deaminase (E.C. 188.8.131.52) was competitively inhibited by 2-azidoadenosine. In the presence of 2-azidoadenosine the enzyme was inactivated by u.v. radiation. The inactivation was diminished in the presence of a protein scavanger.