1 We have investigated the effects of salmeterol (0.3-50 yM) on several pro-inflammatory activities of human neutrophils in vitro. 2 Oxidant production by FMLP-and calcium ionophore (A23187)-activated neutrophils was particularly sensitive to inhibition by low concentrations (0.3-3 IIM) of salmeterol, while the responses of phorbol myristate acetate-and opsonised zymosan-stimulated cells were affected only by higher concentrations (3-50 yM) of the drug. At these concentrations salmeterol is not cytotoxic, nor does it act as a scavenger of superoxide. 3 These anti-oxidative interactions of salmeterol with neutrophils were insensitive to propranolol but could be eliminated by washing the cells, or by pretreatment with low concentrations (1-2 uM) of the pro-oxidative, membrane-destabilizing phospholipids, lysophosphatidylcholine (LPC), platelet activating factor (PAF) and lysoPAF (LPAF). 4 At concentrations of 6.25-50 yM salmeterol interfered with several other activities of stimulated neutrophils, including intracellular calcium fluxes, phospholipase A2 activity and synthesis of PAF. 5 In an assay of membrane-stabilizing activity, salmeterol (25 and 50 Mm) neutralized the haemolytic action of LPC, PAF and LPAF. 6 Of the other commonly used /32-adrenoceptor agonists, fenoterol, and formoterol, but not salbutamol, caused moderate inhibition of neutrophil oxidant generation by a superoxide-scavenging mechanism. However, unlike salmeterol, these agents possessed only weak membrane stabilizing properties. 7 We conclude that salmeterol antagonizes the pro-inflammatory, pro-oxidative activity of several bioactive lipids implicated in the pathogenesis of bronchial asthma, by a mechanism related to the membrane-stabilizing, rather than to the f2-agonist properties of this agent.