Five strains of Haemophilus pleuropneumoniae, out of eight strains tested, produced extracellular haemolysin(s) when grown in liquid culture in the presence, but not in the absence, of RNA. The haemolysin produced by the neotype strain was unstable, heat labile, and sensitive to degradation by pronase, trypsin, and chymotrypsin; moreover, trypan blue treated haemolysin preparations were less effective at causing erythrocyte lysis than were untreated preparations. Following growth in the absence of RNA, washed suspensions of the neotype strain produced extracellular haemolysin when incubated in the presence of RNA, glucose, and casein acid hydrolysate; extracellular haemolysin could not be detected if the incubation mixture contained chloramphenicol. The haemolysin produced by washed bacterial suspensions was similar to that produced by growing cultures in that it was unstable, heat labile, and sensitive to inactivation by the same complement of enzymes. Erythrocyte lysis induced by either haemolysin preparation was preceded by a prelytic phase, the duration of which was dependent upon haemolysin concentration and the initial temperature of the haemolysin--erythrocyte mixture. It is concluded that the haemolysin(s) produced by the neotype strain of H. pleuropneumoniae is distinct from, but closely related to both streptolysin S and the haemolysin produced by Treponema hyodysenteriae.

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