To obtain the subject high-quality derivative in an industrially advantageous method and high yield by passing a solution containing a specific derivative through a step treating with a methacrylic synthetic absorptive resin and a step treating with an aromatic synthetic absorptive resin.

A solution containing an LH-RH derivative is passed through a step treating with a methacylic synthetic absorptive resin and a step treating with an aromatic synthetic absorptive resin to provide the objective derivative. Peptidergic LH-RH derivative (salt) having LH-RH agonist activity and effective for hormone-dependent diseases is exemplified as the LH-RH agonist. A physiologically active peptide (salt), etc., of the formula 5-oxoproline-histidine- tryptophan-serine-tyrosine-Y-leucine-arginine-proline-Z (Y is a residue of D- leucine, D-alanine or the like; Z is NH-C2H5 or glycine-NH2) is exemplified as such derivative.