The effect of Actinobacillus pleuropneumoniae and its metabolites on the viability of porcine alveolar epithelial cells was studied by using a neutral-red uptake test. Alveolar epithelial cells were obtained from 5-week-old colostrum-deprived pigs. The purity of these cells as assessed by the modified Papanicolaou stain was 90 to 95%. Incubation of these cells with 10(6) CFU of a biotype 1 serotype 1 strain resulted in death of the alveolar epithelial cells within 1.5 h. A cytotoxic effect was also seen when alveolar epithelial cells were incubated with sterile culture supernatants of biotype 1 serotype 1, biotype 1 serotype 10, and biotype 2 serotype 2 strains or with ApxI, ApxII, or ApxIII produced by recombinant Escherichia coli. Incubation of alveolar epithelial cells with a knockout mutant of the biotype 1 serotype 1 parent strain which is unable to secrete Apx toxins or with its supernatant did not result in death of these cells. These results indicate that cytotoxicity is at least in part due to production of Apx toxins.