The activity of the steroid-inducible protein lipocortin-1 (LC1; with a primary sequence of 346 amino acids; also called annexin 1), a fragment corresponding to amino acids 1-188 and a short peptide from the N-terminus (amino acid 2-26) were tested for anti-inflammatory actions in three models of acute inflammation in the mouse in comparison with a mAb anti-CD11b (alpha CD11b). In the mouse air-pouch model LC1, fragment 1-188 and peptide Ac2-26 exhibited powerful inhibitory effects (ED50 approximately 5.2, 38 and 88 micrograms/mouse, respectively) on leukocyte migration elicited by IL-1. LC1 was approximately 200 times more potent than Ac2-26 on a molar basis although both gave maximal inhibitions, in contrast fragment 1-188 only produced a partial dose-response curve. LC1 was approximately 20 times more potent on a molar basis in this assay than the alpha CD11b mAb. Peptide Ac2-26 and the mAb alpha CD11b also blocked cell migration into the air-pouch induced by IL-8 with approximately the same potency. In the mouse skin edema and zymosan peritonitis assays Ac2-26 was inhibitory (ED50 of 200 micrograms/mouse) but less so than the alpha CD11b antibody (ED50 approximately 0.5 mg/mouse). Both LC1 (10 micrograms) and Ac2-26 (200 micrograms) completely blocked FMLP-induced neutropenia in the mouse. Studies using an inactivated LC1 preparation, which binds to the same high affinity binding sites as the biologically active material, indicated that the short peptide acts on the same sites as the native LC1. This study confirms the activity of LC1 in another model of experimental inflammation and suggests that it acts partly through inhibition of leukocyte activation with an overall effect qualitatively comparable to the blocking of CD11b portion of a beta 2-integrin complex. It also shows that peptides derived from the N-terminal domain of LC1 may mimic the activity of the full length molecule and points the way for a new family of anti-inflammatory substances that inhibit leukocyte trafficking.