ATK is an analog of arachidonic acid in which the carboxyl group is replaced by a trifluoromethyl ketone group. It inhibits the activity of the 85 kDa cPLA2 and the 80 kDa macrophage iPLA2 without altering the activity of the 14 kDa sPLA2s. ATK is useful for differentiating the effects of cPLA2 and iPLA2 from sPLA2. ATK, at a concentration of 7.5 μM, almost completely inhibits hydrolysis of AEA in a rat brain homogenate.