Abstract: A novel compound (2S,3R,4S)-carboxycyclopropylglycine represented by the formula (1): ##STR1## has proved to be the most potent agonist for the N-methyl-D-aspartate (NMDA) receptor which is one of the glutamate receptor sub-types in the mammalian central nervous system. This compound provides useful tools to open a road to the development of glutamic acid receptor antagonists that may have thereapeutic value in epilepsy, neuronal disorders such as Huntington's chorea and Parkinsonism, as well as various acute and chronic neurodegerative disorders.Also disclosed is a process for producing this compound, as well as a process for individually synthesizing four carboxycyclopropylglycine steroisomers including those of the following formulas (1a, 1b, 1c): ##STR2## and the formula (1).