The haemolysin produced by Listeria monocytogenes at 37 degrees C and 4 degrees C was examined in fertile hens' eggs. Organisms, cell free extracts and purified haemolysin derived from broth cultures grown at the lower temperature were more pathogenic for chick embryos, induced higher mortality with toxic changes in the embryos. These effects were most pronounced with the purified haemolysin as shown by LD50 determinations and following inoculation of constant haemolytic doses. Pathological changes induced by the haemolysin included sub-cutaneous haemorrhage due to endothelial damage, hepatosplenomegaly with macroscopic and histological lesions in heart, spleen and liver in the absence of an inflammatory response. At the cellular level, the myocardial tissue, and hepatocyte structure were destroyed with intravascular haemolysis, fatty degeneration of mitochondria, dilation of endoplasmic reticulum and distortion of liver cell nuclear membranes evident. The mortality and morphological data showed an increase in virulence for Listeria after culture at 4 degrees C compared with 37 degrees C and suggested a more cytotoxic component of the haemolysin which was activated at lower temperatures.