Using immunofluorescent antibody techniques, conversion of pyruvate kinase (PK) isozymes during the maturation of erythroblasts was studied. In normal subjects, M2-type PK was clearly seen at the proerythroblast stage, then markedly declined with cell maturation, whereas L-type PK continued to increase. In classical type PK deficiency, M2-type PK was still clearly seen in orthochromatic erythroblasts, whereas L-type PK was scarcely detected during maturation. In other PK deficiency cases, change of L-type PK showed a similar pattern to that of normal while M2-type PK was still clearly seen at the later stage of maturation as in the classical type. In K-562 human leukemia cell line, M2-type PK declined and L-type PK increased in parallel with hemoglobin synthesis by addition of hemin, an inducer for differentiation into erythroblasts. Hemoglobin synthesis and L-type PK production appear to have interrelation during the differentiation and maturation of erythroblasts.