Eight structural analogues of oxygenated triterpenes exerted striking differences in activation of human platelets. They arc four pairs of stereoisomers and two pairs of positional isomers with varying: 1) acetoxyl/hydroxyl substituents; 2) the position of the substituents at C-3 and C-15; and 3) the stereochemistry of a substituent at C-3. It required a threshold concentration for each agent to cause the concentration-dependent activation. These triterpenes were hydrophobic with < 20% difference in the partition coefficients between 1-octanol and water. They caused differential effects on: inositol triphosphate production; the increase in [Ca2+]i; diacylglycerol formation; phosphatidic acid accumulation, protein phosphorylations and arachidonate release. These agents activated both phospholipases C and A2. The trend of activating phospholipase C was triterpenes with two acetoxyl substituents > one acetoxyl/one hydroxyl substituents > two hydroxyl substituents. In activating phospholipase A2, triterpenes with two acetoxyl substituents were most effective, whereas the paired isomers with a hydroxyl group at C-15a and an acetoxyl substituent at C-3 failed the activation. The results enable one to discuss the possible structure-activity relationship of various oxygenated triterpenes in the activation of both phospholipases C and A2.