The flavonoid quercetin is a potent inhibitor of calcium- and phospholipid-dependent protein kinase (Ca, PL-PK) activity from mouse brain. Half-maximal inhibition of the kinase occurs at about 10 microM. If the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) is used instead of calcium as a stimulating factor of the kinase enzyme activity is still inhibited by quercetin. The kinase inhibitor, however, does not interfere with the binding of TPA to its receptor.