An enzyme system of Mycobacterium smegmatis catalyzing the elongation of medium-chain fatty acids with acetyl-CoA was obtained free from de novo fatty acid synthetase by ammonium sulfate fractionation. The system was resolved by gel filtration and DEAE-cellulose chromatography into three fractions, all of which were required for reconstitution of the elongation activity. The three fractions were highly purified enoyl-CoA hydratase, highly purified 3-hydroxyacyl-CoA dehydrogenase, and a fraction containing both enoyl-CoA reductase and thiolase. The reconstituted system was avidin-insenstive, required NADH as a sole hydrogen donor, and was sensitive to pCMB, but not to N-ethylmaleimide or monoiodoacetate. Decanoyl-CoA and octanoyl-CoA were the best primers for the elongation system. When decanoyl-CoA was used as the primer, the major product was found to be a lauroyl derivative (probably lauroyl-CoA). Evidence was obtained suggesting that acyl-CoA dehydrogenase, catalyzing the first step of beta-oxidation, was not functional in the elongation system.