Estrogen-receptor binding moieties were introduced into Pt(II) complexes in order to facilitate the selective transport into cancer cells. Estradiol esters of 2,3-diaminopropionic acid and estradiol ethers of 1,2-diamino-2-methyl-3-(p-hydroxyphenyl)propane were attached to Pt(II) complexes. The antitumor activity of the compounds was tested towards the human mammary carcinoma cell lines MDA-MB 231 and MCF-7, respectively, and the estrogen-receptor binding affinity of the Pt(II) complexes was determined. The steroidal Pt(II) complexes gave a maximum growth inhibition of 80% and a maximum estrogen-receptor binding affinity of 5.18.