An antagonist of NMDA-sensitive glutamate receptors in rat brain membrane and rat spinal motoneurones was isolated from Amanita pantherina and ideneified as a diastereoisomeric mixture of 2-amino-3-(1, 2-dicarboxyethylthi)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 properties.