1 We have developed a novel model of allergen-induced eosinophil extravasation into mouse air-pouches following sensitization and challenge with ovalbumin (Ova). This model was used to investigate the mechanism(s) underlying the anti-in¯ammatory action of the glucocorticoid hormone dexamethasone (Dex). 2 Injection of 10 mg Ova into 6-day-old dorsal air-pouches of mice sensitized to the same antigen provoked an intense cell accumulation as early as 6 h post-challenge (0.08+0.03 and 4.0+1.0610 5 leucocytes in saline and Ova-treated air-pouches, respectively), maximal at 24 h (0.02+0.01 and 6.0+0.8610 5 leucocytes in saline and Ova-treated air-pouches, respectively) and persisted up to 48 h. At the 24 h time-point, the cellular in®ltrate consisted of 37% eosinophils, 18% neutrophils and 45% mononuclear cells, as assessed by histological examination. The same ratio of eosinophil/neutrophil was obtained by¯uorescence-activated cell sorting (FACS) analysis, since 72% of the polymorphonuclear (PMN) population was positive for very-late antigen-4 (VLA-4) expression. 3 Subcutaneous (s.c.) administration of Dex (50 or 100 mg per mouse, 71 h) inhibited eosinophil accumulation into Ova challenged air-pouches by about 70% (P50.05) and 75% (P50.05), respectively, when compared to controls. Cell accumulation measured at 48 h after Ova injection was also signi®cantly reduced (775%) by Dex administration at the 24 h time-point (n=12, P50.05). 4 Eosinophil numbers in the bone marrow and blood were quantitated. We found that the sensitization protocol induced a 3 fold increase in eosinophil numbers in the bone marrow (naive mice: 2.7+0.3610 5 ; sensitized mice: 8.7+1.7610 5 , P50.05) and blood (naive mice: 0.5+0.2610 5 ; sensitized mice: 1.5+0.3610 5 , P50.01). However, 24 h following Ova challenge, the eosinophil numbers in the bone marrow had dropped (3.7+0.8610 5 ) with no change in the circulating pool, suggesting an equilibrium within the eosinophil pools had been reached. 5 Dex administration provoked a profound eosinopaenia in the blood of naive (5.2+1.5 to 0.9+0.6610 4 ) and sensitized mice (1.5+0.3 to 0.08+0.02610 5 ) at 4 h. This e ect was reversed within 24 h. Dex also inhibited the release of eosinophils from the bone marrow in response to Ova challenge. 6 We show for the ®rst time that eosinophils express the steroid-inducible protein lipocortin 1 (LC1). FACS analysis of eosinophils emigrated into the Ova challenged air-pouches revealed detectable LC1-like immunoreactivity (373610 3 ). These data were also substantiated by LC1 detection in circulating eosinophils of interleukin-5 transgenic mice (strain: CBA/Ca). However, s.c. injection of Dex (50 mg) did not alter LC1 levels in blood eosinophils, such that 235+21610 3 LC1-like molecules per cell were measured after vehicle treatment (n=5), and 224+8610 3 LC1-like molecules per cell were associated with this cell type 1 h after steroid treatment (n=5, not signi®cant). Finally, resident eosinophils (in the pleural cavity) were found to have much higher LC1 levels than ...