Partially purified glucose-6-phosphate dehydrogenase was isolated from small amounts of human erythrocytes (15-20 ml). The Km value for glucose-6-phosphate was 35.0 +/- 3.0 micronM, the Km for NADP was 4.27 +/- 0.3 micronM. The optimal activity of the enzyme was at pH 9.0. Glucose-6-phosphate dehydrogenase, dialyzed in presence of 1-10(-5) M NADP, had critical temperature about 52 degrees within 10 min of incubation; without NADP it was at 45 degrees. The method for isolation and purification of the enzyme was modified.