Quantitative estimations of the yellow enzyme (flavin) in tissues have been only approximate. A more exact (colorimetric) method of determination of the content of liver, kidneys, heart, brain and lungs, is described; other tissues (spleen, blood, etc.) which contain other pigments cannot be estimated by this method. The flavin content of the organs of a normal cat and a normal rabbit is reported. In avitaminosis-A the content of the liver was less than normal (4 rats and 1 calf). In vitamin C deficiency of guinea pigs (5 livers and 3 kidneys) there was no clear-cut picture; a high protein diet (15 rats) also gave no definite alteration in the flavin content of the liver.