S U M M A R YCell-wall fractions from Mycobacterium lepraernurium were examined chemically and immunologically. The major sugar and amino acid components detected were arabinose and galactose, alanine, glutamic acid, a,€-diaminopimelic acid (DAP) and hexosamine, but substantial amounts of aspartic acid, glycine, valine, serine, threonine, leucine and iso-leucine were also present in preparations not extracted with neutral lipid solvents and alkaline ethanol. However, after such extraction and treatment with proteolytic enzymes, alanine, glutamic acid and DAP (I -5 : 1-0 : 1.0) were present in much larger amounts than any other amino acids (not more than 0-1 5), indicating that these three are the mucopeptide constituents. Cell-wall agglutination tests indicated the presence of an antigen shared with other mycobacteria.