The efficacy of two inactivated vaccines containing the Apx toxins of Actinobacillus pleuropneumoniae (Hemopig, Biokema, Lausanne, Switzerland and Porcilis App, Intervet, Boxmeer, the Netherlands) was determined. Ten pigs were vaccinated twice with Hemopig and eight pigs with Porcilis App. Ten control animals were injected twice with a saline solution. Three weeks after the second vaccination, all pigs were endobronchially inoculated with 10(5) colony-forming units (CFU) of an A. pleuropneumoniae serotype 9 strain. Increased respiratory rate and/or fever were observed in all vaccinated and control pigs after challenge. One pig of the Hemopig group and of the Porcilis App group died, whereas all pigs of the control group survived the challenge. Surviving pigs were killed at 7 days after challenge. The mean percentage of affected lung tissue was 34% in the control group, 16% in the Hemopig group, and 17% in the Porcilis App group. A. pleuropneumoniae was isolated from the lungs of all 10 control animals, from 7 of the 10 animals vaccinated with Hemopig and from 5 of the 8 animals vaccinated with Porcilis App. The mean bacterial titres of the caudal lung lobes were 1.4 x 10(6) CFU/g in the control group, 1.7 x 10(3) CFU/g in the Hemopig group, and 4.8 x 10(3) CFU/g in the Porcilis App group. In both vaccinated groups the mean number of days with dyspnoea, the mean number of days with fever, the mean percentage of affected lung tissue, and the mean bacterial titre in the caudal lung lobes were significantly lower than in the control group. Significant differences between the two vaccinated groups were not observed. It was concluded that both vaccines induced partial protection.