Smear-ripened cheeses are frequently contaminated with Listeria monocytogenes. The establishment of defined ripening/protective cultures containing antilisterial yeasts and staphylococci would be ideal. For a commercial application, it is of major importance to identify the substance responsible for the antilisterial activity. The antilisterial activity of the P. norvegensis strain WSYC 592 was only observed when the yeast strain and L. monocytogenes grew in a co-culture. Various experiments using the cell-free yeast culture supernatant did not show any inhibitory activity at all. It is likely that P. norvegensis produces one or more unstable antilisterial substances that have to be synthesized constantly in order to inhibit Listeria. The Staphylococcus equorum strain WS 4524 produces an antilisterial substance that is stable against heat and enzymes. Furthermore, the presence of a peptide synthetase gene indicates similarity to Micrococcin P₁, a well-known peptide antibiotic.