Protein kinase D (PKD) is a member of the protein kinase C (PKC) superfamily with distinctive structural, enzymological and regulatory properties. Identification of the cellular function(s) of PKD has been hampered by the absence of a selective inhibitor. Recently, Stewart et al. showed that resveratrol inhibited PKD, but not various PKC isoforms, in vitro. Here we confirmed that the activity of PKD is indeed inhibited in vitro by resveratrol (IC(50) approximately 200 microM). Additionally, we assessed the inhibition by resveratrol of PKD activity in intact cells, by Western blotting with a phosphospecific PKD antibody which recognizes the autophosphorylated enzyme. In this setting, very high concentrations of resveratrol were required to achieve inhibition of PKD autophosphorylation (IC(50) approximately 800 microM). Since resveratrol produces other pharmacological effects (e.g., cyclooxygenase inhibition) at lower concentrations than those required to inhibit PKD in intact cells, its value as a selective tool to investigate the cellular function(s) of PKD is questionable.