Extraction of fruitbodies of Agaricus xanthodermus with sodium sulfite solution yielded the antibiotically active 4-hydroxybenzenediazonium ion in form of its stable sulfonate. In the course of our screening for antibiotically active metabolites from basidiomycetes, we found that extracts of fruitbodies of A. xanthodermus contained an antimicrobial, cytotoxic, antineoplastic, and prophage-inducing metabolite. The specimen, A. xanthodermus, belongs to the widely distributed poisonous mushrooms. Now we wish to report on the occurrence of the 4-hydroxybenzenediazonium ion (1) in the extracts of A. xanthodermus. This diazonium ion is responsible for the antibiotic activity and could be isolated in form of its stable sulfonate which we named agaridin (2).