The main properties of a monofunctional riboflavin kinase from B. subtilis have been studied for the first time; the enzyme is responsible for a key reaction in flavin biosynthesis--the ATP-dependent phosphorylation of riboflavin with production of flavin mononucleotide. The active form of the enzyme is a monomer with molecular weight of about 26 kD with a strict specificity for reduced riboflavin. To display its maximum activity, the enzyme needs ATP and Mg(2+). During the phosphorylation of riboflavin, Mg(2+) could be partially replaced by ions of other bivalent metals, the efficiencies of which decreased in the series Mg(2+) > Mn(2+) > Zn(2+), whereas Co(2+) and Ca2+ had inhibiting effects. The flavokinase activity was maximal at pH 8.5 and 52 degrees C. ATP could be partially replaced by other triphosphates, their donor activity decreasing in the series: ATP > dATP > CTP > UTP. The Michaelis constants for riboflavin and ATP were 0.15 and 112 micro M, respectively. As compared to riboflavin, a tenfold excess of its analog 7,8-dimethyl-10-(O-methylacetoxime)-isoalloxazine decreased the enzyme activity by 30%. Other analogs of riboflavin failed to markedly affect the enzyme activity.