Our gonadotropin-releasing hormone (GnRH) antagonist analogue MI-1544 ([Ac-D-Trp1,3,D-Cpa2,D-Lys6,D-Ala10]GnRH) was developed as a potential contraceptive material, because it decreased the luteinizing hormone level without unfavourable side-effects. The antagonist was covalently bound to poly[Lys-(Ac-Glu0.96-DL-Ala3.1)] (AcEAK)-a branched polypeptide having a polylysine backbone--resulting in a MI-1544-AcEAK conjugate. According to our in vitro experiments the MI-1544 induced a 33%-35% decrease in cell numbers of MCF-7 and MDA-MB-231 human breast cancer cell lines at a dose of 30 microM. The biodegradable polymeric carrier, AcEAK, alone inhibited cell proliferation by only 13%-15%, while the MI-1544-AcEAK conjugate, applied at the same dose, was capable of producing 45%-50% inhibition of cell proliferation. Our in vivo experiments using immunosuppressed mice showed that MI-1544, applied twice daily s.c., inhibited the growth of oestrogensensitive and -insensitive xenografts by 65% and 30% respectively. This effect was potentiated (70%) in both types of xenografts by the presence of the polymeric carrier in the conjugate; however, the carrier by itself did not cause tumour growth inhibition. The polymeric polypeptide carrier is supposed to increase the stability of the GnRH antagonist and to prevent the rapid excretion of the covalently bound peptide molecule. The antagonist and its conjugate may have various direct and indirect effects on breast cancer cells and, as a consequence, the new GnRH antagonist conjugates are suitable for treating an extended range of breast cancers.