AbstractConversion of pyruvate kinase (PK) isozymes during the maturation of erythroblasts in five cases of PK deficiency was compared with that in normal subjects using immunofluorescent antibody techniques. In normal erythroblasts, M2‐type PK was clearly seen at the proerythroblast stage, then markedly declined with cell maturation whereas L‐type PK continued to increase. In two patients with classical type PK deficiency, M2‐type PK was still clearly seen in orthochromatic erythroblast, whereas L‐type PK was hardly detected during maturation. In three patients with other types of PK deficiency, change of L‐type PK was similar to that of normal subjects whereas M2‐type PK was clearly seen even at the later stage of maturation as in classical‐type PK deficiency. The present studies indicate that compensatory M2‐type PK production occurs in the erythroblasts of patients wit