Thirty-five flavonoids of seven different types, namely isoflavonoids, chal
cones, dihydroflavonols, flavanols, flavanones, flavones, and flavonols wer
e investigated for their ability to inhibit ascorbate-induced microsomal li
pid peroxidation and their cytotoxicity. For each activity a structure-acti
vity relationship was established. Subsequently, an antioxidant selectivity
index, i.e., the maximal non-toxic dose divided by the IC50 value for lipi
d peroxidation, was introduced. Kaempferol showed the highest antioxidant s
electivity index of all flavonoids tested.