Abstract Adenosine deaminase (from calf intestinal mucosa) converts 8,2′-anhydro-8-mercap to-9-β-D-arabinofuranosyladenine (II)) into 8,2′-anhydro-8-mercapto-9-β-D-arabinofuranosylhypoxanthine (III) with Km of 2.0 × 10−4M and V max equal to 7% that of adenosine. This conversion serves as a useful preparative synthesis of III. Further the fact that II (where the purine base is locked in the anti conformation) is a substrate for the enzyme while 8-bromoadenosine (where the base is in the syn conformation) is not a substrate supports the idea that substrates for adenosine deaminase must exist in the anti conformation.