We have previously reported that Rikko-san (RKS) inhibited the lipopolysaccharide (LPS)-stimulated prostaglandin (PG) E2 in mouse macrophage-like RAW264.7 cells without affecting the expression of cyclooxygenase (COX)-2. Here RKS inhibition of the enzyme activity of both COX-1 and COX-2 proteins was investigated. Western blot analysis showed that RKS did not significantly change the S-nitrosylated COX-2 protein level. On the other hand, RKS inhibited the PG production catalyzed by purified COX-2, more effectively than that catalyzed by purified COX-1. These results suggest that RKS inhibits the PGE2 production by selectively inhibiting the COX-2 activity in activated macrophages.