Structure-related inhibitory effects on the prostaglandin synthesis in vitro have been detected among eight diterpenoids of the cembrane class (1-8). The 4,6-diols 1 and 2 are the most active of the cembranoids studied with IC50 values of 0.36 and 0.39 mM, respectively, as compared with an IC50 value of 1.4 for acetylsalicylic acid. Lower activities are found for cembranoids having undergone oxidation of/at the 11,12 double bond (5-8), while the 4-hydroxy-6-oxocembranoids 3 and 4 exhibit effects lower than those of 1 and 2 but higher than those of 5-8.