A kinetic equation for methionine formation from homosysteine with B_<12>-dependent methionine synthetase was deduced from the well-known scheme proposed by Weissbach and Taylor. The appropriateness of the equation was examined experimentally in the reaction system with the enzyme from Escherichia coli 215 cultivated in the presence of CN-B_<12>. The action of various corrinoids on the methionine formation was evaluated with respect to the kinetic parameters involved in the equation. CH_3-B_<12> showed a high affinity for the enzyme and a high catalytic activity for formation of holoenzyme compared with those of CN-B_<12>. DBCC had a low activity for methionine synthesis, while fluoromethyl phosphito-P-cobalamin, a high activity. 2-Amino-2-methyl-1-propanol cyanocobalamin and cobalt free corrinoid strongly inhibited the reaction.