A process for separating the diastereomers of RSS— and SSS—N-α[1-carboxy-3-phenylpropyl]lysylproline is described. Previous chromatographic processes for separating the diastereomers of this peptide active substance, such as for example adsorption chromatography, exhibited disadvantages with regard to the solvents used and throughput. These disadvantages do not occur if basic ion exchangers are used in the chromatographic separation of the diastereomers. In particular, purely aqueous solutions may be used as the eluent.
