Monoclonal antibodies (MAbs) were raised against extracellular hemolytic and cytotoxic proteins of Actinobacillus pleuropneumoniae serotypes 2 and 9. MAbs were tested for inhibition of hemolytic and cytotoxic activities of the reference strains of A. pleuropneumoniae serotypes 1 to 12. Five MAbs inhibited hemolytic or cytotoxic activities of one or more serotypes and were used to probe Western blots (immunoblots) of cytotoxic and hemolytic culture filtrates of serotypes 1 to 12. The MAbs recognized three different proteins: (i) a protein of approximately 103 kDa that was associated with a weak hemolytic activity and a moderate cytotoxic activity, (ii) a protein of approximately 105 kDa that was associated with a strong hemolytic activity and a strong cytotoxic activity, and (iii) a protein of approximately 120 kDa that was associated with a strong cytotoxic activity but not with hemolytic activity. Serotypes 6, 7, and 12 produced only the 103-kDa protein, and serotype 10 produced only the 105-kDa protein. The other serotypes produced two proteins: serotypes 1, 5, 9, and 11 produced the 103- and 105-kDa proteins, and serotypes 2, 3, 4, and 8 produced the 103- and 120-kDa proteins.