SEVERAL micro-organisms are known to cause opacity when growing in fluid or on solid culture media containing native, non-heated egg-yolk^{1â 13}. This egg-yolk reaction can help in the classification of the bacteria and in some species it is associated with the virulence (especially toxicity) of the organisms. We have observed that Listeria monocytogenes strains regularly produced opacity in egg-yolk agar medium hitherto not reported in the literature.