Vitamin B_<12> in a human serum can be determined by improved technique utilizing two mutant strains of E. coli, one of which, #215,responds to either vitamin B_<12> or methionine and the other, #1544,to methionine or homocystine. (a) Only negligible amounts of methionine are uncovered in a serum after autoclaving it with KCN solution in a acetate buffer of pH 4.6. (b) On the other hand, a large amount of methionine are recovered in a serum treated with papain prior to autoclaving. In this case, vitamin B_<12> and methionine may be assayed after separating them by means of paper chromatography using water saturated n-butanol as a solvent. Results obtained with ten samples were as follows : (a) Serums untreated with papain ; 0.279±0.012mγ/ml. The recovery of vitamin B_<12> added to samples ranged from 87 to 102 per cent. (b) Serums treated with papain ; 0.300±0.025mγ/ml. The recovery of vitamin B_<12> added to samples ranged from 80 to 115 per cent.