Partially purified glucose-6-phosphate dehydrogenase (G6PD) was studied in erythrocytes of patients with hereditary hemolytic anemia. Kinetic and electrophoretic properties of G6PD distinctly varied both in patients--homozygotes and in maternal heterozygotes. All the enzymes studied together with the decreased activity possessed the reduced Km value for NADP and G6P. Anomalous enzymes were shown to differ from normal ones by several patterns (pH-dependency, thermostability, % of the substrate analogues utilization, electrophoretic mobility etc). The mutant variant of G6PD was not described earlier. The anomalies variant was called "Kremenchug" to indicate the place of proband origination.