Methylmalonyl-CoA decarboxylase of Veillonella alcalescens catalyzed the isotopic exchange between methylmalonyl-CoA and [1-14C]propionyl-CoA or between malonyl-CoA and [1-14C]acetyl-CoA. The exchange was independent of sodium ions and was abolished by avidin. The enzyme also catalyzed the carboxyl transfer reaction from methylmalonyl-CoA to acetyl-CoA yielding propionyl-CoA and malonyl-CoA, and vice versa. The beta subunit was dissociated from methylmalonyl-CoA decarboxylase by prolonged washing of the enzyme while bound via its biotin prosthetic group to monomeric avidin-Sepharose. The beta-chain-depleted enzyme was inactive as a methylmalonyl-CoA decarboxylase but retained carboxyltransferase activity. The beta subunits were specifically protected by Na+ ions from tryptic hydrolysis. Based on these and other observations the following functions may be assigned to the different polypeptide chains of methylmalonyl-CoA decarboxylase: carboxyltransferase (alpha), carboxybiotin-carrier-protein decarboxylase (beta), biotin carrier protein (gamma). The function of the delta chain is unknown.