Cytokines are known to increase the production of prostaglandins by human decidual cells, but negative regulators have not been identified. We have examined the effects of dexamethasone and progesterone on prostaglandin (PG) E2 synthesis by cultured human first trimester decidual cells. The numbers of cyclooxygenase (COX) enzyme positive cells were visualised by immunocytochemistry, using antibodies specific for COX-1 and COX-2. Interleukin-1 beta stimulated the production of prostaglandins E2 and F2 alpha dose-dependently, and this was associated with increased numbers of COX-2 positive cells. Progesterone (10(-7)-10(-6) M) and dexamethasone (10(-7)-10(-6) M) inhibited basal and interleukin-1 beta-stimulated prostaglandin production, and decreased the numbers of COX-2 positive cells. Neither interleukin-1 beta nor the steroids affected numbers of COX-1 positive cells. COX-2 seems to be the main enzyme controlling the synthesis of PGE2 by human decidual cells, and may be negatively regulated by progesterone.