The role of the heat-labile haemolysin of Actinobacillus pleuropneumoniae in acute porcine pleuropneumonia was examined. A virulent strain was compared with an isogenic haemolysin-deficient mutant in experimental infections. The pigs which received the virulent strain showed clinical signs of acute respiratory disease whereas the animals infected with the mutant strain appeared to be less severely affected. At post mortem examination, both groups showed similar acute pulmonary lesions and pleurisy typical of A pleuropneumoniae infection. The bacterial antigen representing the haemolysin was detected in lung lesions infected with the parent strain but not in those infected with the mutant. These results demonstrate that the haemolysin of serotype 2 A pleuropneumoniae is not an essential factor for the production of the lesions of pleuropneumonia in pigs.