The enhancement of nitric oxide (NO) production in glycyrrhizin (GL)-induced macrophages (Mφ) in response to lipopolysaccharide (LPS) was investigated. NO production in GL-induced macrophage culture supernatants was stimulated in response to LPS (10 μg/ml) for 24-or 48-h cultures, and these levels were compared three times with the levels in saline-induced peritoneal exudate cell cultures. Furthermore, Mφ induced with proteose peptone (PP) containing GL could generate greater NO production than Mφ induced with PP alone. However, no stimulation of NO production was observed by addition of GL in the cultures of Mφ induced with thioglycollate or Bacillus Calmette Guerin. Moreover, GL-induced Mφ showed cytostasis against such tumor target cells as L 1210 and P 388 lymphoma cell lines. These observations indicate that GL can activate the Mφ in vivo system and stimulate NO production in response to LPS.