1. 1. Mycobacterium tubercolosis, BCG, Mycobacterium smegmatis, Mycobacterium sp. strain Rabinowitch, and Mycobacterium sp. strain 521, defatted by ethanol-ether and chloroform were treated with phenol and trypsin. The residual fractions contained 35-40% of species-specific mycolic acid, 33-40% of neutral sugars, 7-8% of amino sugars and 12-18% of amino acids. The neutral sugars were D-arabinose and D-galactose which form an arabinogalactan. Amino sugars and amino acids (glucosamine, muramic acid, alanine, glutamic acid and α-ε{lunate}-diaminopimelic acid) formed a mucopeptide. 2. 2. The linkage between arabinogalactan and mucopeptide was demonstrated by ion-exchange chromatography and electrophoresis of the arbinogalactan-mucopeptide complex. 3. 3. The ester linkage between mycolic acid and arabinogalactan was demonstrated by isolating the mycolic acid-sugar complex by means of mild acid hydrolysis. 4. 4. The mycolic acid-arabinogalactan-mucopeptide complex may be a common structure of mycobacterial cell walls. © 1968.