A soluble exotoxin (a leucocidin) which was lethal to peripheral blood leucocytes from cattle, sheep, rabbits and man (in order of decreasing sensitivity) was elaborated by a variety of isolates of Fusobacterium necrophorum when the majority of organisms were present as filaments in liquid culture. Maximum production of the leucocidin was achieved by concentrations of bacteria equivalent to between 4 X 10(7) and 4 X 10(8) short cells per ml of culture above which no further increase in titre was observed. The ability of different batches of medium to support production of leucocidin was reflected in their capacity to enable F. necrophorum to grow to this range of concentration. Prolonged culture of the organism, resulting in a decline to below 6 in the pH of the medium was associated with a depression in the titre of leucocidin, presumably due to its inactivation under these conditions. The leucocidin was stable at 4 degrees C for at least 30 days, to extremes of pH (4 to 9) for 1 h at room temperature, and showed maximum activity in assays conducted at 37 degrees C at pH 7 to 8. The exotoxin was inactivated by heating at 56 degrees C for 30 min and possessed a molecular weight around 250,000 to 300,000 as determined by gel filtration and membrane partition chromatography.