The invention provides compounds of the general formula:
(in which R1 is the residue of glycine or D-alanine, and all other amino-acid residues are in the L-form, X1 and X2, which may be the same or different, are OH or NH2 and n is 0 or 1) and their physiologically acceptable salts.
The compounds of the invention are capable of inhibit- ing the myelopoietic system of humans and animals, thereby protecting said system against attack by cytotoxic drugs.