A case of chronic non-spherocytic hemolytic anemia due to partial erythrocyte phosphofructokinase deficiency (61% of normal) is reported. Immunological studies in hemolystates, using anti-muscle and anti-leukocyte phosphofructokinase antisera, seemed to indicate that an isozyme of the muscle type was deficient in the patient. This hypothesis was confirmed by the studies of muscle phosphofructokinase; this enzyme was an unstable and fast variant. There was no deficiency in muscle because of the active synthesis of proteins by this tissue, but the deficiency could be detected in erythrocytes, old cells which are no longer able to synthesize proteins.