Protaminobacter ruber was cultured in a medium containing [57Co]cyanocobalamin with a "two-step cultivation method" and the forms of vitamin B12 compounds in the cells were examined. Methylcobalamin was detected in the early phases of growth and reached a maximum of about 40% of all cobalamins extracted from the cells. In the stationary phase of growth, almost all cobalamins consisted of adenosylcobalamin. Recultivation of the cells of the stationary phase in a fresh medium resulted in the conversion of adenosylcobalamin into methylcobalamin. Interconversion of methylcobalamin and adenosylcobalamin was presumed from these facts. The formation of adenosylcobalamin from methylcobalamin was demonstrated with a cell-free extract system from P. ruber. The rate of conversion of methylcobalamin into adenosylcobalamin was highest among several cobalamin analogs tested. Propylation of 5-methyltetrahydrofolate: homocysteine methyltransferase with 1-iodopropane did not affect this conversion reaction, which was probably catalyzed by methyltransferase and adenosyltransferase.