A malonyl-CoA-dependent, acyl carrier protein (ACP)-non-requiring fatty acid elongation system was isolated from Mycobacterium avium. Chromatographical fractionation and reconstitution studies indicated that the system is composed of separable protein components like a fatty acid elongation system from M. smegmatis reported previously. In comparison of the two systems, however, some distinct features were observed in pyridine-nucleotide-coenzyme requirements and in sensitivities to isoniazid (N-amino-3-pyridine carboxylic acid amide); the system from M. avium used NADPH as a sole hydrogen donor and was inhibited by the agent, in vitro, at the concentration of 2mM, while the agent has been proved to have no inhibitory effect on the system from M. smegmatis which required both NADH and NADPH. Using the fractionated component enzymes of the system from M. avium, similar concentrations of isoniazid were found to be distinctly inhibitory against 3-oxoacyl-CoA reductase and more slightly against enoyl-GoA reductase with Ki values of 352.8 μM and 5.5 mM, respectively.