The cooperative and antagonistic effect of extracellular bacterial proteins of Streptococcus agalactiae, Rhodococcus equi, Corynebacterium ovis, and Corynebacterium haemolyticum with proteins of listerial strains of various sources, species and serovars of the membrane of sheep erythrocytes was investigated. Listeria monocytogenes and Listeria ivanovii produce, beside listeriolysin, further proteins. Their specific effect on the sheep erythrocyte membrane becomes apparent after the appearance of substances produced by the organisms under study. Ten strains of L. ivanovii produced phospholipase C responsible for the zone of incomplete haemolysis. It was inhibited by the non-haemolytic sphingomyelinase D of C. ovis and C. haemolyticum. Chromatographic analysis revealed that phospholipase C splits sphingomyelin in the membrane of sheep erythrocytes. The inhibition of double haemolysis of L. ivanovii by sphyngomyelinase D of C. ovis on agar plates with washed (!) sheep erythrocytes can be utilized as a specific and rapid identification test of L. ivanovii.