1. Increased levels of several pro-inflammatory cytokines including interleukin-1 beta (IL-1 beta) and tumour necrosis factor-alpha (TNF alpha) have been found in bronchoalveolar lavage fluid from symptomatic asthmatic patients. IL-1 beta, TNF alpha and interferon-gamma (IFN gamma) are known to stimulate a number of cells to produce inflammatory mediators such as prostaglandins. Although airway smooth muscle (ASM) is known to be a rich source of prostaglandins, the regulation of cyclo-oxygenase (COX) isoforms and prostanoid production by proinflammatory cytokines have not been studied in human airway smooth muscle. 2. We studied the effects of IL-1 beta, TNF alpha and IFN gamma on the induction of two isoforms of cyclo-oxygenase and its relation to prostaglandin E2 (PGE2) release and COX activity (reflected by PGE2 synthesis from exogenous arachidonic acid) in human cultured airway smooth muscle cells. 3. IL-1 beta, but not TNF alpha or IFN gamma, caused a time- and concentration-dependent enhancement in PGE2 and other prostanoid (6-keto-PGF1 alpha, PGF2 alpha, thromboxane B2 (TXB2) and PGD2) production, with PGE2 and 6-keto-PGF1 alpha as the principal products. This stimulation was accompanied by a corresponding increase in COX activity. 4. COX-2 protein measured by Western blot analysis was not detectable in untreated cells, but was increased in a time- and concentration-dependent manner by IL-1 beta, but not TNF alpha or IFN gamma. In contrast, no variation in the expression of COX-1 protein was observed. 5. Pretreatment with the conventional non-steroidal anti-inflammatory drugs (NSAIDs), indomethacin and ibuprofen, and the selective COX-2 inhibitors, NS-398 and nimesulide, completely blocked IL-1 beta-induced PGE2 release and COX activity. The glucocorticosteroid dexamethasone and protein synthesis inhibitors, cycloheximide and actinomycin D, not only markedly inhibited IL-1 beta-stimulated PGE2 release and COX activity but also suppressed IL-1 beta-induced COX-2 induction. 6. This study demonstrates that human cultured ASM cells release prostanoids in response to IL-1 beta stimulation and that the response is mostly mediated by the induction of COX-2 rather than COX-1 isoenzyme, implying that airway smooth muscle may be an important source of prostaglandins in human airways and that COX-2 may play an important role in the regulation of the inflammatory process in asthma.