Polymorphonuclear cell (PMN) infiltration is a hallmark of ricin-induced mucosal inflammation, yet the cellular processes involved in initiating this reaction remain undefined. In this study we report that ricin stimulates the human monocyte/macrophages cell line 28SC to secrete IL-8, a potent PMN chemoattractant. IL-8 release in response to ricin was both dose- and time-dependent. 28SC cells did not secrete IL-8 when exposed to formaldehyde-inactivated holotoxin or ricin B subunit. Furthermore, IL-8 induction could be blocked by brefeldin A, which inhibits ricin translocation into the cytosol. As predicted from the literature, we observed elevated levels of p38 mitogen activated protein kinase (MAPK), a post-transcriptional regulator of IL-8, in 28SC cells as early as 3h after ricin exposure. Treatment of 28SC cells with the pyridylimidizole analogue SB203580, a known inhibitor of p38 MAPK, suppressed ricin-mediated IL-8 release. We conclude that ricin stimulates human monocyte/macrophages to produce IL-8 by activation of the p38 MAPK pathway, raising the possibility that p38 MAPK inhibitors may potentially serve as therapeutic agents to suppress mucosal inflammation associated with ricin intoxication.