1Methanogenic bacteria with pseudomurein sacculi incubated with elevated concentrations of amino acids showed growth inhibition, changes in morphology and, under certain conditions, lysis. Methanobacterium therrnoautotrophicum incorporated the amino acids glycine, threonine, ornithine and, in small amounts, aspartic acid into the sacculi, but not D-alanine, mesodiaminopimelic acid or the amino sugar galactosamine.