Specific monoclonal antibodies (MoAbs) to Actinobacillus pleuropneumoniae (APP) serotypes 1 and 2 which recognized serotype-specific antigens were produced. It was revealed that the two serotype-specific MoAbs H1-18 and H22-7 recognized O polysaccharides of the lipopolysaccharide (LPS) from APP serotypes 1 and 2, respectively, in the results of antigen analysis by means of SDS-PAGE and Western blotting. Furthermore, ddY mice immunized passively with the above type-specific MoAbs were protected against challenge infection by the homologous serotype of APP at 24 or 48 hr later. However, H1-9 and H3-2 MoAbs recognizing serotype-common protein antigens of APP did not show signs of any protective effect. These results showed that LPS from APP bacterial cells was one of the structural substances in the serotype-specific antigens, and an important component as one of the antigens protecting against the homologous serotype strain.