Objective: Adhesion molecules of the beta 2 integrin family (CD11/CD18) play a pivotal role in neutrophil (PMN) trafficking. Ligand binding of the integrins activates the non-receptor tyrosine kinase syk that phosphorylates the mammalian actin-binding protein 1 (mAbp1). Here we studied the role of syk and mAbp1 for PMN trafficking. Methods: Adhesion, Spreading and migration was measured in neutrophil-like differentiated (dHL- 60) cells or murine PMN from Syk bone marrow chimeric mice and control chimeras or mAbp1 /- and mAbp1+/+ mice. Intravital microscopy of cremaster muscle venules was performed 2 h after stimulation with tumor necrosis factor alpha. Microflow chambers (VitroCom) or IBIDI m-slides were used to study adhesion, spreading and crawling of murine PMN under flow conditions. Results: Whereas PMN adhesion, spreading and migration were severely compromised in syk-/- PMN under static conditions in vitro when compared to controls, the genetic absence of mAbp1 had no effect. In contrast, mAbp1 was indispensable for leukocyte adhesion and spreading in N-formyl-Met- leu-Phe-stimulated cremaster muscle venules of mice and in a flow chamber coated with P- selectin, ICAM-1 and keratinocyte-derived chemokine (KC, CXCL1). Moreover, mAbp1-/- PMN revealed a significant reduction of the crawling velocity under flow conditions compared to control cells. This result was confirmed using downregulation of mAbp1 by RNA interference technique in dHL-60 cells. In addition, control dHL-60 cells performed crawling perpendicular to the direction of flow as expected, whereas downregulation of mAbp1 abrogated perpendicular cell migration and led to cell movement in the direction of flow. Conclusion: mAbp1 seems to be of fundamental importance for PMN adhesion, spreading and crawling under physiological shear stress conditions. Taken together, we identified mAbp1 as a novel player in the molecular machinery allowing PMN trafficking under flow conditions. Supported by DFG (Wa 1048/2-3).