E-selectin is a membrane protein expressed by endothelial cells activated by cytokines released during the inflammatory process; it plays an important role in neutrophil emigration into inflamed tissues. To further explore in vivo the function of E-selectin, we have generated transgenic mouse line expressing E-selectin under the control of a chicken beta-actin promoter. In these mice, the number of blood neutrophils was reduced, without any other obvious phenotype or tissue damage. These neutrophils, however, displayed two significant changes: first, an alteration in the levels of expression of two membrane receptors involved in neutrophil adhesion to endothelial cells, namely a marked increased in the Mac-1 antigen (CD11b/CD18) and a decrease in the Mel-14 antigen (L-selectin); second, an increased oxidative activity when compared to blood neutrophils of non-transgenic mice, as shown by their capacity to oxidize 2',7'-dichlorofluorescein (DCFH) into a fluorescent compound. These observations indicate that the binding of E-selection with neutrophils bearing its ligands promotes neutrophil activation in vivo.