In the present study, we have investigated the participation of resident peritoneal cells (macrophages and mast cells) in the neutrophil migration induced in rats by the intraperitoneal administration of LTB₄, fMLP or C5a des arg. The intraperitoneal injection of LTB₄ (10 nmol), fMLP (10 nmol) and C5a des arg (zymosan-activated plasma, 1 ml) caused an intense neutrophil migration compared to the saline control (1,000, 1,500 and 2,000%, respectively). An 83% depletion in the number of resident cells following peritoneal lavage reduced the LTB₄-stimulated neutrophil migration by 73.6% without affecting that caused by fMLP and C5a des arg. Increasing the peritoneal macrophage population (236%) by pretreating the cavities with thioglycollate enhanced the neutrophil migration induced by LTB₄ (129%), but did not alter that induced by fMLP and C5a des arg. Similarly, reducing the population of peritoneal mast cells containing toluidine-blue-staining granules by subchronically pretreating the cavities with compound 48/80 diminished the LTB₄-induced NM by 69% but had no effect on the responses to fMLP and C5a des arg. Pretreating the animals with dexamethasone strongly inhibited (70%) the neutrophil migration induced by the intraperitoneal injection of LTB₄, fMLP and C5a des arg. Indomethacin, BW A4C and NDGA had no such effect. The incubating medium from peritoneal macrophages and mast cells stimulated with LTB₄ induced neutrophil migration when injected into the peritoneal cavity of rats. This migration was strongly reduced (70%) by treating the cells with dexamethasone. In contrast, stimulating the cells with fMLP or C5a des arg did not result in the release of any promigratory activity into the incubating fluid. Our results suggest that LTB₄ induces neutrophil migration via a mechanism dependent on resident mast cells and macrophages while that induced by C5a des arg and fMLP seems to be independent of such cellular involvement. The neutrophil migration induced by LTB₄ is apparently mediated by factor(s) whose release is blocked by dexamethasone. fMLP and C5a appear to cause in vivo migration by the formation of a concentration gradient and by a glucocorticoid-sensitive mechanism different from that stimulated by LTB₄.