[en] Z-17α-iodovinyl-11β-chloromethyl-estradiol (Z-CMIV), a new selective estradiol derivative, can easily be labeled with high efficiency by radioactive iodine isotopes. Biodistribution studies and quantitative scintigraphic imaging of human breast carcinoma xenografts in mice demonstrated continuous and selective accumulation of the [¹²³I]Z-CMIV, in estrogen receptor (ER)-positive target tumors, with significantly high target/nontarget ratio up to 48 h post-injection. A receptor-mediated mechanism of concentration of Z-CMIV in target tissues was confirmed by scintigraphic imaging and by biodistribution studies