Mycobacterium avium accumulates 14 C-palmitic acid with saturation kinetics; the process is both temperature dependent and p H sensitive. The fatty acid is incorporated into triglyceride in vivo and the conversion is detectable within 5 min after exposure of the cells to 14 C-palmitic acid. The triglyceride is rapidly utilized because 14 CO 2 evolution from it begins within 30 min after 14 C-palmitic acid accumulation. Data from silicic acid column chromatography of extracts of cultures that have divided many times in medium containing 14 C-palmitic acid indicate that a large proportion of the cell lipid is triglyceride, but the radioactivity is widely dispersed among the other lipids. It is estimated that about 5% of the cell dry weight is triglyceride in a postexponential culture.