Abstract: The present invention relates to a method of detecting biologically active substances affecting intracellular processes, an isolated DNA, a vector and a cell for use in the method. More specifically, the present invention provides a method for detecting a biologically active substance that affects intracellular processes mediated through a protein kinase or a second messenger, which comprises incubating a cell with a test substance and measuring any change caused by the test substance in the flourescence of (i) a wild-type or modified green flourescent protein (GFP) having a protein kinase recognition site, (ii) a modified GFP containing a second messenger binding domain, or (iii) a hybrid polypeptide having a wild-type or modified GFP and an attached protein kinase recognition site or a second messenger binding domain.