Inflammation is usually a tightly controlled process which confines tissue damage, prevents infection, and assists in cellular regeneration. However, if the inflammatory response becomes unregulated, this normally beneficial local event may escalate into a wider malignant activity, characterized by endothelial injury, excessive cell infiltration, and vascular leakage. Due to the ability of platelet-activating factor and tumor necrosis factor to elicit the release of each other, 'prime' cell responses, and influence the activity of other cytokines, we propose that these two mediators play a pivotal role in the formation of deleterious feedback cycles leading to the above endothelial damage which may underlie pathologies such as shock, sepsis, ischemia, and asthma. Platelet-activating factor antagonists such as BN 52021 inhibit the priming and other effects induced by platelet-activating factor and thus may be of therapeutic value in such conditions.