AbstractNormal or increased amounts of series III porphyrins with greater amounts of series I were observed on incubation of PBG in hemolysates of congenital erythropoietic porphyria vs. normal erythrocytes, human or bovine. Correlation with reticulocyte percentage was poor, in the aggregate a general trend toward increased values of both isomers I and III was noted with increasing reticulocytes. When the percent of type III was low the net amount was increased as compared with normal. Hemolysates of non‐porphyric, reticulocyte‐rich red cells (hemolytic or posthemorrhagic anemia) formed only minute amounts of type I porphyrin but at the same time no more, or even less type III than the porphyric hemolysates, although representing red cells of greater reticulocyte content. No evidence of deficient heme synthesis was observed in porphyric hemolysates incubated with [14C] ‐porphobilinogen or59Fe. Other studies of porphyric hemolysates incubated with and without added mouse spleen synthetase failed to reveal evidence of an absolute UPG‐III cosynthetase (Co‐S) deficiency. The large increases of type I porphyrin with normal or increased formation of type III, both in the disease and in the hemolysates, are believed due to a primary increase of ALA‐S or UPG‐S activity rather than a decrease of Co‐S. Possible mutations which might be responsible for this increase