Nine Danish Actinobacillus pleuropneumoniae biotype 1 isolates were shown by latex agglutination and indirect haemagglutination to possess capsular polysaccharide epitopes identical to those of serotype 2 strain 1536 (reference strain of serotype 2) and strain 4226 (Danish serotype 2 strain). Immunodiffusion confirmed the antigenic relationship with serotype 2 and further demonstrated an antigenic relationship with strain WF83 (reference strain of serotype 7). SDS-PAGE with LPS from strains 1536, 4226, WF83 and strain 7317 (representative of the 9 isolates examined) showed that strains WF83 and 7317 had an identical smooth ladder pattern whereas LPS from strains 1536 and 4226 showed a distinctly different pattern. The antigenic similarities of the LPS of strains WF83 and 7317 were confirmed by immunoblots using rabbit or pig antisera prepared against the 3 strains. No antigenic similarities in the LPS of strains 1536 and 7317 were revealed. Since an antigenic determinant specific for the 9 isolates could not be demonstrated with the methods used, the strains are proposed to be designated K2:O7.