A 100-200-mesh methoxybenzhydrylamine resin was used as a solid carrier. The first amino acid was introduced on the resin, and all 28 amino acids of peptide A were subsequently coupled, by the carbodiimide method. The a-amino function was protected with BOC group, and the side chains were blocked with the following groups: Asp, cyclohexyl; Ser and Thr, benzyl; and Tyr 2-bromobenzyloxycarbonyl. Boc(Fmoc)Lys were added at positions 11, 12, 20, 21 and 29 to have at further steps the analog contg. hArg in these positions. Boc was removed with 55% TFA in dichloromethane, and Fmoc was removed with 50% piperidine in DMF After Fmoc was removed from Lys, the substituted guanidine group was introduced by the reaction with NN% bis(tert-butyloxycarbonyt)-S-methylisothiourea in the presence of 4-(N,N-dimethylamino)pyridine. All Boc were removed from the fully protected peptidyl polymer by the reaction with trifluoroacetic acid. The remaining protecting groups left the peptide while it was cleaved from the resin with liq. HF The HF was removed, the residue was washed with Et2O, and crude peptide was extd. with 50% AcOH and lyophilized. The acetate form was obtained on an ion-exchange resin and preparative HPLC gave 99.7% pure A. Polysulfonic filters were used during the preparation of the stable and reproducible pharmaceutical form of A, viz., an aq. lyophilizate (phosphate buffer) contg. 100 mu g peptide A and mannitol per ampule. Peptide A was about a few score times as bioactive as the hGH-RH reference peptide, completely nontoxic in doses up to 120 ig and resistant to enzymatic degradation, increased the level of hGH and the no. of somatotropic cells in animal pituitary glands, and affected no other pituitary hormones.