Protein kinase C (PKC) modulating activity of 81 plant extracts of Finnish origin was investigated with an automated bioassay method combined to LC/MS. Twenty-one extracts from different parts of the plants inhibited PKC significantly. Fractionation of the active extract of Filipendula ulmaria showed that this method was able to identify a PKC inhibiting compound from the extract as quercetin. Our results indicate that this method is suitable for PKC screening of complex matrices and provides a quick and low volume, non-radioactive, alternative method for PKC experiments.