Thirty-one type A strains of Pasteurella multocida isolated from rabbits and poultry were able to agglutinate red blood cells (RBC) from human group O donors. Except for human RBC from group B which were also agglutinated by 3 strains, neither group A nor RBC from sheep and rabbits were agglutinated. Haemagglutination was mannose-resistant, and the two techniques used detected unrelated activities. One as tested by a slide haemagglutination (MRSH) test was common to most (31 out of 34 strains) capsulated strains, though non-mucoid ones obtained by growth on hyaluronidase-containing medium showed also this property. This haemagglutination activity was destroyed at 100 degrees C for 15 min but not at 56 degrees C for 30 min, was dependent upon an optimal pH range (7.4-7.5) and did not seem to be influenced by different enriched media. The other activity was assayed by the microhaemagglutination (MRMH) test. Only part (19 out of 34 strains) of the strains presented this activity, which was also destroyed by boiling for 15 min and was dependent on the addition of 0.5 of polyvinilpirrolidone to phosphate buffered saline, used to suspend RBC. A correlation between haemagglutination activities of P. multocida type A and the possibility of this microorganism harbouring fimbriate organelles is discussed.