The congenital alterations of red cell enzymes are one of the most frequent causes of red cell hemolysis. The enzyme defects have more important effects on the red cells than any other cells in the body with the same alteration. It is because red cells do not have a protein regenerating system, this meaning that they have to keep the enzyme level during the circulation time. In order to study this defect, they are classified as follows: a) defects of glycolytic pathway; b) defects of glutathionE++ pathway; c) defects of the nucleotidic metabolism. The most frequent defects are: the deficiency of: G6PD, of pyruvate kinase, of glucose phosphate isomerase and probably pyrimidine 5'-nucleotidase deficiency.