“Fourth flavin compound”, which had been found in heated liver homogenate, was formed from purified FAD by heating its aqueous solution. The conditions for optimum formation of FFC from FAD were found as follows: 1. Optimum pH existed at between pH 4 and 8. 2. FFC formation was increased with ascending temperature until 130°. 3. The formation of FFC decreased with increasing concentration of FAD. Under the optimum conditions, FFC was produced from FAD, purified and crystallized.
