Since Warburg and Christian described the "yellow enzyme" in the year 1932, a number of papers have been published concerning riboflavin, which is an important part of the yellow enzyme. Chemists and biologists, however, were more successful in isolating riboflavin and in defining its various properties than physicians were in establishing its clinical value. No clinical observations to speak of had been published at the time when I reported on the possibility of a photodynamic action of vitamin B2, in 1936.1 Not much attention had been paid to two reports published in the previous years. Von Euler and Adler2 were the first to report on remarkable amounts of flavin in the retina of certain fishes. In some cases they found as much as 50 mg. per hundred cubic centimeters, the highest concentration to be found in any tissue. The flavin found in the retina is not bound