A multi-faceted approach was used to investigate the effect of an anti-inflammatory peptide derived from human lipocortin 1 N-terminus region (amino acid 2-26; termed human Ac2-26) on human neutrophil activation in vitro. When incubated with purified human neutrophils. human Ac2-26 produced a concentration-dependent inhibition of elastase release stimulated by formyl-Met-Leu-Phe (fMLP), platelet-activating factor, or leukotriene B4, with an approximate EC50 of 33 microM (100 micrograms/ml). At this concentration, human Ac2-26 also inhibited (77%) the release of [3H]-arachidonic acid from neutrophils stimulated with fMLP. The peptide, however, did not inhibit the up-regulation of the beta 2-integrin CD11b and the concomitant shedding of L-selectin from neutrophil plasma membrane induced by fMLP. In adhesion experiments, human Ac2-26 inhibited neutrophil adhesion to endothelial monolayers when this was stimulated with fMLP, but not when this followed endothelial cell activation with histamine or platelet-activating factor. Again, the effect of the peptide was concentration-dependent, and an approximate EC50 of 33 microM was calculated. When a preparation of 125I-labeled human Ac2-26 was incubated with the neutrophils, the peptide was internalised in an energy-dependent fashion. All together, these observations lead us to propose a model in which this peptide derived from the N-terminus of human lipocortin 1 alters a common cellular mechanism producing a selective inhibition of neutrophil activation.