γ-Glutamyl-marasmine, a new natural dipeptide containing an unusual cysteine sulphoxide moiety has been isolated from the Basidiomyceteous mushrooms Marasmius alliaceus, M. scorodonius and M. prasiosmus, which are known for their garlic like odour. It is shown that this compound is the common natural precursor and that its two step enzymatic cleavage leads to the odorous substances. In the first step γ-glutamyl-marasmine is cleaved by a γ-glutamyl transpeptidase. The formed marasmine is split in a second enzymatic reaction by a C-S lyase into pyruvic acid, ammonia and an unstable sulfur compound, which decomposes to form the odorous secondary products.