The testosterone-estradiol binding protein (sex binding protein = SBP), immunopurified from human placental blood, was photolabelled by irradiation at lambda greater than 300 mm in the presence of tritiated 17 beta-hydroxy-androsta-4,6-dien-3-one. High-performance reverse-phase liquid chromatography of tryptic peptides, showed two main peaks of radioactivity. Sequence determination of these two fractions indicated that the radioactivity was associated with an undetectable amino-acid preceded either by the sequence His-Pro-Ile (major peak) or Arg-His-Pro-Ile at the N-terminal site and bearing Arg as C-terminal amino-acid. Comparison with the sequence reported for human SBP (K.A. Walsh et al., Biochemistry, 25, 1986, pp. 7584-7590) suggested that radioactive labelling was localized on the Met-139 residue of the hexapeptide Arg-His-Pro-Ile-Met-Arg (fragment 135-140).