SFKs are required for PMN firm adhesion to adherent platelets under conditions of physiologic flow. (A) Human PMNs, pretreated with different concentration of PP2, PP3, or vehicle (DMSO) for 10 minutes, were perfused across a platelet surface at a flow rate of 2 dynes/cm² for 2 minutes. Then, the chamber was perfused with fresh media for an additional 2 minutes at 5 dynes/cm², without interruption in the perfusion. The interaction of PMNs with adherent platelets was observed by contrast phase video microscopy with a 20×/0.40 NA objective. The total number of cells interacting at 2 minutes and the fractions of firmly adhered PMNs in the last 20 seconds of flow were measured using an ad hoc software for image analysis as described in “Materials and methods.” The results presented are means ± SEM from 5 different experiments performed using cells from different donors. *P < .05 for treatment versus control. (B) PMNs were isolated from bone marrow of wild-type, hck^−/−fgr^−/−, or hck^−/−fgr^−/−lyn^−/− mice. Flow experiments were performed exactly as described for human cells. The results presented are mean ± SEM for 6 different experiments each performed in triplicate with cells pooled from 5 wild-type or knock-out mice. *P < .05 for wild-type versus hck^−/−fgr^−/−lyn^−/− cells.