The participation of corrinoids in the biosynthesis of CH3-S-CoM and CH4 from CH3OH by extracts of heterotrophically and autotrophically grown cells of Methanosarcina barkeri was investigated. Alkyl-B12 derivatives severely inhibited the transfer of the Co-methyl moiety of CH3-B12 to HS-CoM, but hardly affected the formation of CH3-S-CoM and CH4 from CH3OH. 1-Iodoalkanes completely suppressed methanogenesis from CH3OH, but only slightly inhibited the biosynthesis of CH3-S-CoM. Additionally, [methyl-3H]methylcobalamin could not be detected in cell-free extracts following biosynthesis of [methyl-3H]CH3-S-CoM from HS-CoM and excess [methyl-3H]CH3OH. These results indicate that methylcobalamin-HS-CoM methyl-transferase is not involved in the biosynthesis of CH3-S-CoM or CH4 from CH3OH by M. barkeri.