Abstract: This invention relates to a process for preparing carbamyl derivatives of .alpha.-hydroxy acids by the hydrolysis of 5-substituted 2,4-oxazolidinediones.The carbamyl derivatives can be further hydrolyzed to .alpha.-hydroxy acids.The invention also relates to a process for the enzymatic hydrolysis of racemic 5-substituted 2,4-oxazolidinediones to give only one of the two possible optical isomers, i.e. the D-carbamyl-.alpha.-hydroxy acid. The free D-.alpha.-hydroxy acid can be obtained from the optically active carbamyl derivative by simple hydrolysis. Of particular interest is the case in which the D-.alpha.-hydroxy acid is D(-)mandelic acid. The enzymatic activity required for preparing the carbamyl derivative of D(-)mandelic acid has been found both in homogenized veal liver and in a series of microorganisms, including Agrobacterium radiobacter, Bacillus brevis, Bacillus stearothermophilus, Pseudomonas sp., Pseudomonas desmolytica, Pseudomonas fluorescens, Pseudomonas putida.