THE PRESENCE of free protoporphyrin in the circulating red blood cells was first described in 1928 by van den Bergh and Hyman.1This discovery was widely confirmed, and in the intervening period considerable knowledge has been gained of variations in concentration in disease.2In 1936 it was pointed out that reticulocytes are especially rich in porphyrin.3At that time a satisfactory method of quantitative determination was not available. Later, with the use of the method devised in collaboration with Grinsterin,4it soon became apparent that the correlation of erythrocyte porphyrin concentration with reticulocyte percentage varied considerably with the situation at hand.2aIn acute phenylhydrazine anemia of rabbits the correlation was quite close; in pernicious anemia with liver therapy the peak of porphyrin concentration in the red cells was considerably later than the peak of reticulocyte percentage; in iron deficiency anemias and lead poisoning very great