The haematological mechanisms in the course of liver abscess formation were evaluated. They were examined by employing viable cells of Fusobacterium necrophorum subsp. necrophorum and Fusobacterium necrophorum subsp. funduliforme in comparison with their endotoxins. Whole cell infection with F.n. necrophorum led to neutrophilia and to a concomitant monocytosis in parallel with those responses induced by the in vivo injection of its endotoxin. Viable infection with F.n. funduliforme was characterized by a sustained endotoxin-related monocytosis against neutropenia. The stimulatory impact of endotoxin on monocytes when released from a viable F.n. funduliforme infection suggested an inherently peculiar mechanism which differed from the induction of both neutrophilia and monocytosis when F.n. funduliforme endotoxin was administered alone. The neutrophilic inducing capacity of the F.n. necrophorum endotoxin was equally illustrated by its positive chemotactic effect on polymorphonuclear neutrophils in vitro. The data presented here emphasize the virulence of F.n. necrophorum viewed in reference to changes in leucocyte trafficking and as complemented by a relatively high endotoxin content.