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Glycogen phosphorylase of human leucocytes exists in two forms interconvertible by phosphorylation-dephosphorylation. The active form of the enzyme from normal and leukemic human leucocytes has been obtained by preincubation of the 14,000 g crude extracts at 30 degrees C in the presence of ATP and magnesium ion. The enzyme associated itself to a glycogen particulate fraction obtained by centrifugation at 90,000 g from the crude extracts. Kinetic characteristics of the glycogen fraction enzyme were similar when obtained from normal or leukemic leucocytes. Apparent Km values of the active enzyme for glucose-1-phosphate and glycogen were 2-3 X 10(-3) M and 0.50-0.65 mg/ml respectively. Glucose, glucose-6-phosphate, uridine diphosphoglucose, 2-deoxyglucose, fluoride, Mg+2 and Ca+2 have been shown to be inhibitors of the enzyme.