In accordance with the present invention there are provided methods for producing enantiomerically pure .alpha.-substituted carboxylic acids, such as, for example, .alpha.-amino acids and .alpha.-hydroxy acids, said method comprising combining an aldehyde or ketone with a cyanide and ammonia or an anmionium salt or an amine, in the presence of a nitrilase or a polypeptide having nitrilase activity which stereoselectively hydrolyzes the amino nitrile or cyanohydrin intermediate under conditions sufficient to produce the enantiomerically pure .alpha.-substituted carboxylic acid, such as, for example, .alpha.-amino acids and .alpha.-hydroxy acids.