During our investigation of the neuroprotective activity of Platycodi radix we found that an aqueous extract of this folk medicine exhibited significant protection against glutamate-induced toxicity in primary cultured rat cortical cells. In order to clarify the neuroprotective mechanism(s) of this observed effect, activity-guided isolation was performed to seek and identify active fractions and components. By such fractionation, four known triterpene saponin compounds--platycodins A, C and D and deapioplatycodin D--were isolated from the n-butanol fraction. Among these four compounds, platycodin A exhibited significant neuroprotective activities against glutamate-induced toxicity, exhibiting cell viability of about 50%, at concentrations ranging from 0.1 microM to 10 microM. Therefore, the neuroprotective effect of Platycodi radix might be due to the inhibition of glutamate-induced toxicity by the saponin compounds it contains.