The level of flavin compounds (riboflavin, FMN, FAD) in green leaves will be controlled mainly by the levels of the enzymes which are concerned in vivo biosynthesis of flavins (riboflavin synthetase, riboflavin kinase, FAD pyrophosphorylase), and hydrolysis of FMN (acid phosphatase). These four enzymes localized in the cytoplasm of cell. In spinach, the activity of acid phosphatase was about 1000 times as much as that of riboflavin kinase (Table 1) in vitro. However, it was found that the hydrolysis of FMN was extremely retarded by the nucleotides, inorganic phosphate and metallic ions (Table 3,4 and Fig. 5). Moreover, the low affinity of FMN for the acid phosphatase and low content of it in green leaves may lead to decrease the rate of FMN hydrolysis in vivo. The properties of acid phosphatase concerned with the enzymatic hydrolysis of FMN are as follows : (1) Optimum pH value is near 5.5. The activity markedly decreases above pH 7.0 where the activities of the biosynthetic enzymes are optimum. (2) The Km value for FMN is 2.0×10^<-4>M. (3) The enzyme was inhibited by several metallic ions, especially with Zn^<2+> which is one of the essential factors for riboflavin kinase. (4) Nucleotides such as AMP, ADP, ATP, GTP, CTP or ITP and inorganic phosphate are potent competitive inhibitors. The authors also found that the enzymatic hydrolysis of inosine-5'-monophosphate (IMP) which was catalyzed by the 5'-nucleotidases of microorganisms was significantly inhibited by the presence of a small amounts of ATP. This observation may be worth to stabilize IMP added to manufactured food by the addition of nucleotides including ATP.