Flavokinase isolated from 18-day chick embryo liver shows optimal activity at pH 8.6, in the presence of the divalent cations Zn2+, Ca2+ and Mn2+. 2. The activity and stability of flavokinase depends on the RBP concentration. 3. Interaction between flavokinase and riboflavin binding protein was demonstrated by means of crossed immunoelectrophoresis and affinity chromatography.