Coagglutination and ring precipitation tests were used to study the effect of heat on the surface antigens of Haemophilus pleuropneumoniae strains employing the reference strains belonging to serotypes 1 to 7 and field isolates belonging to serotypes 1, 2, 3, 5, 6 and 7. By immunising rabbits with formalin-fixed whole-cell suspension, antibodies were obtained which sensitised Cowan I Staphylococcus aureus to coagglutinate antigen preparations which had not been heated, or heated at 56 degrees C, or boiled or autoclaved. Similar positive reactions were obtained with the ring precipitation test. Heating the cultures at 56 degrees C for one hour was best for exposing the most potent serotype-specific antigens in all the strains studied. All the reference strains and most of the field isolates possessed the thermostable type specific antigens which could withstand autoclaving for one hour. However, many field isolates belonging to serotype 1 did not possess this antigen. The apparent antigenic heterogeneity of serotype 1 strains based on the presence or absence of these thermostable antigens could be valuable in epidemiological investigations. It was shown that most potent serotype-specific antigens are present as freely diffusible material on the surface layer of the bacterial cells, which could easily be removed by washing in saline solution. Well washed bacterial cells devoid of surface materials are poor antigens. It is recommended that test strains should not be heated above 56 degrees C for serotyping because higher temperatures are liable to destroy the capsular antigen of some strains and render the culture untypeable.