An antagonist of NMDA-sensitive glutamate receptors in rat brain membrane and rat spinal motoneurones was isolated from Amanita pantherina and identified as a diastereoisomeric mixture of 2-amino-3-(1,2-dicarboxyethylthio)propanoic acids. The mixture was separated and the absolute configurations of the components were determined as (2R), (1'R) and (2R), (1'S), by analysis of the optical properties.