Objective - To investigate rheologic properties of bovine neutrophils that may result in adhesion molecule-independent sequestration of neutrophils in inflamed lungs of cattle. Animals - Healthy 2-to 4-week-old male Holstein calves. Procedures - Neutrophil deformability, filamentous actin (F-actin) content, and CD11b expression was determined for unstimulated bovine neutrophils and bovine neutrophils incubated with the inflammatory mediators tumor necrosis factor-α (TNF), platelet-activating factor (PAF), interleukin-8 (IL-8), zymosan-activated plasma (ZAP), Pasteurella haemolytica-derived lipopolysaccharide (LPS), and P haemolytica leukotoxin. Neutrophils were separated into 3 subpopulations on the basis of size.The F-actin content and CD11b expression were evaluated by use of flow cytometry. Leukocyte deformability was evaluated by filtration of dilute whole blood. Results - The subpopulation of the smallest-sized neutrophils (> 90% of neutrophils) contained little F-actin. A subpopulation of slightly larger neutrophils had a profound increase in F-actin content and CD11b expression. The subpopulation of the largest neutrophils had increased F-actin content and CD11b expression, compared with those for both subpopulations of smaller neutrophils. Incubation of neutrophils with PAF and ZAR but not TNF, IL-8, LPS, or leukotoxin, resulted in decreased neutrophil deformability and increased F-actin content. Incubation with PAF and TNF induced an increase in size of neutrophils. Conclusions and Clinical Relevance - Size can be used to identify subpopulations of large and rigid neutrophils in blood samples from healthy calves. Platelet-activating factor and activated complement fragments are potent inducers of F-actin formation and neutrophil rigidity. Physical changes in neutrophils may impede their transit through lung microvasculature and result in leukocyte trapping independent of adhesion molecule interactions with endothelial cells. (Am J Vet Res 2000;61:380-386).