Neutrophil activation has been thought to contribute to the pathogenesis of fibrinous pneumonia caused by Pasteurella haemolytica serotype A1. The activation of bovine neutrophils by culture fluid from the pathogenic P. haemolytica serotype A1 and the non-pathogenic serotype A11 was compared. Logarithmic-phase bacteria of each serotype were incubated in RPMI 1640-medium for 3 h at 37 degrees C. The culture fluid was collected by centrifugation and concentrated by ultrafiltration. The concentrated culture fluids were then tested for their ability to induce chemotaxis and respiratory burst in bovine neutrophils. Chemotactic activity was of similar magnitude in response to both serotypes. An early chemiluminescence response occurred at 5 min at 1:100 dilution and a late peak at 11 min at 1:500 dilution for serotype A1. The early peak was absent at all dilutions tested for serotype A11. Maximal chemiluminescence response was observed at 1:25 dilution with serotype A11 while maximal response was seen at 1:500 dilution with the culture fluid from serotype A1. Superoxide anion release was greater in response to culture fluid from A1 than A11 at all dilutions tested. Leukotoxin activity was 50-fold higher in culture fluid from serotype A1 than in culture fluid from serotype A11. In this study, the ability of P. haemolytica to attract and activate bovine neutrophils was not restricted to the pathogenic serotype A1.