1. 1. Isoelectric focusing in polyacrylamide gels identified three erythrocytic electrophoretic patterns of purine nucleoside phosphorylase in a survey of 16 mouse strains. 2. 2. Three strain-specific electrophoretic types were also evident in liver, kidney and spleen leukocytes. 3. 3. There are 3-fold differences in purine nucleoside phosphorylase activities between strains for several tissues; C57BL/6J and Mus spretus having the greatest and the least activity, respectively. 4. 4. Within strains there were up to 8-fold tissue-specific differences in activity with the order from greatest to least being: liver, kidney, spleen leukocyte, erythrocyte, heart.