The potential use of monocins for listeria typing was investigated. Monocins are defective phage particles still capable of lysing listerial cells. They were induced by u.v.-irradiation, precipitated with polyethylene glycol and purified by density gradient centrifugation. Using 26 monocins, it was possible to type 48% of Listeria monocytogenes strains, 92% of L. innocua strains and 94% of L. ivanovii strains. Overall typability of 480 strains was 68%. None of the monocins was able to lyse L. grayi. Monocin typing was found to be a valuable supplementary tool for typing strains which were non-typable by the Weihenstephan phage typing set. A combination of phage typing and monocin typing increased overall typability of Listeria strains to 95%.