Numerous reports have appeared concerning the study of a variety of enzymes present in the erythrocyte. Only a few (e.g. (l-6)), however, have been prepared and investigated in a significantly purified state. Because the erythrocyte contains little non-hemoglobin protein, it would appear to be an excellent source for the preparation of certain enzymes which it contains at relatively high concentrations. Appropriate procedures will be described in this and subsequent papers for the purification of various enzymes of the human erythrocyte. The present report will describe methods for isolating, in highly purified form, a purine nucleoside phosphorylase from the human erythrocyte. The ability of crude hemolysates to degrade adenosine was described in the early literature by Dische (7). A limited study of the action of hemolysates on adenosine was included as part of a previous report (8), at which time it was suggested that adenosine degradation is mediated by the action of a nucleoside phosphorylase which acts on the deaminated product (inosine) . Purification of the phosphorylase was undertaken in order to investigate its specific properties. Purification of a purine nucleoside phosphorylase from liver tissue has been described by Kalckar (9), who has contributed much of the available information on this enzyme (e.g. (10, 11)). Purification of specific pyrimidine nucleoside phosphorylase from bacteria (12) as well as mammalian tissue (13) and the preparation of both a purine nucleoside phosphorylase and hydrolase from yeast (14) have also been previously reported.