For studying the relationship between their haemolysin producing capacity and pathogenicity on mice, 12 Listeria strains were investigated, and 9 of them were reinvestigated one year later. Haemolysin production was measured on 51Cr-labelled sheep erythrocytes and the 50% haemolysis-inducing dilution of the bacterial suspensions was calculated. Pathogenicity of the strains was estimated by intraperitoneal injection of mice. In both experimental series the correlation coefficients were calculated (r = 0.408, t = 1.41 and r = 0.163, t = 0.43). Using this experimental system, no correlation existed between the haemoysin producing capacity and mouse-pathogenicity of these strains.