The effects of sinomenine (7,8-didehydro-4-hydroxy-3,7-dimethoxy-17-methyl- 9 alpha,13 alpha,14 alpha-morphinan-6-one), a pure alkaloid extracted from the Chinese medical plant Sinomenium acutum, on different macrophage capacities were investigated in vitro using resident mouse peritoneal macrophages and the macrophage-like cell line RAW 264.7. Sinomenine markedly decreased prostaglandin E2 and leukotriene C4 synthesis of macrophages stimulated by zymosan or calcium ionophore and also significantly inhibited the nitric oxide production of RAW 264.7 cells activated by interferon-gamma/lipopolysaccharide. It can be considered that these effects are part of the analgesic, anti-inflammatory, and antirheumatic mechanisms of sinomenine.