Synnematin, a new antibiotic produced by a sp. Tilachlidium (Cephalosporium). is soluble in water, methanol, 75% acetone and 50% ethanol and is labile to acid and heat. It was purified by adsorption on 1% Darco-G 60, eluted with 75% acetone and clarified with 2% Nuchar. Its activity is slightly greater in acid than in alkaline media and resistant strains are not developed very rapidly. It inhibits most strains of Brucella. Coryne-bacterium. Micrococcus. Streptococcus. Salmonella and some strains of Shigella but has low activity against most strains of Aerobacter. Escherichia. Mycobacterium and the filamentous molds tested. In vivo activity against Diplococcus pheumoniae infection in mice and S. pullorum infection in chick embryos was demonstrated. It has a low toxicity for mice.