Prenylated flavonoids isolated from Erythrina senegalensis were evaluated for their ability to inhibit PLCgamma1 activity in vitro and the formation of inositol phosphates (IPt) in PLCgamma1-overexpressing NIH3T3 fibroblast cells (NIH3T3gamma1 cells). These flavonoids inhibited PLCgamma1 activity in a dose-dependent manner (IC (50) values ranged from 1.0 microg/mL to 35.0 microg/mL) with the exception of alpinumisoflavone ( 5). Also erysenegalensein D ( 2) and erysenegalensein N ( 3) showed inhibition of inositol phosphate formation in NIH3T3gamma1 cells with IC (50) values of 13.0 microg/mL and 10 microg/mL, respectively. Analyzing the relation of structure to activity, the number of prenyl groups and the hydroxylation of the prenyl groups were important factors for the inhibition of PLCgamma1 activity and of inositol phosphate formation.