Studies on the toxic effects of streptolysin 'O': electrocardiographic changes in mammals. 
The effect of streptolysin 'O' (a streptococcal exotoxin) on ECG in rabbits and mongrel puppys has been investigated.  The intravenous administration of streptolysin 'O' (SLO) in doses of 0.5 to 2 U/kg in rabbits and 0.5 to 4 U/kg in puppys immediately caused transient sinus bradycardia, which was dose dependent.  Intense sinus bradycardia followed by ST elevation and progressively increasing atrioventricular blocks in rabbits and nodal rhythm and intraventricular conduction defects in puppys, terminating in cardiac standstill were observed after a dose of 4 and 8 U/kg respectively.  Pretreatment of rabbits with antistreptolysin 'O' completely protected them against the cardiotoxic and lethal effects of challenging doses of SLO.  It is suggested that this streptococcal component may be involved in the causation of derangements of cardiac activity during acute rheumatic fever in man.