Abstract In addition to the classical estrogen receptor, chick oviduct cytosol contains a sex steroid binding component (SSB) with specificity for steroidal estrogens, androgens and progestins. We have optimized the measurement of SSB and have further characterized this protein. It was possible to quantitate [ 3H]estradiol binding to SSB by performing the measurements in the presence of excess diethylstilbestrol, which saturates the estrogen receptor and does not bind to SSB, and by using excess progesterone to determine nonspecific binding. Since SSB appears to be quite unstable with rapid hormone dissociation kinetics, we determined that short incubation times (usually 2 h) at 0°C with 20–30% glycerol in the buffer gave optimal SSB measurements. The affinity of SSB for estradiol ( K d = 20 nM) is about 5% that of the estrogen receptor. In addition to estradiol, several androgens and progestins bind to SSB. However, the nonsteroidal antiestrogen, H1285 does not bind to SSB even though it binds well to the avian estrogen receptor. The tissue content of SSB is about 15-fold greater than for estrogen receptor and is stimulated by estrogen treatment. Whereas labeled SSB cannot be readily resolved by ion-exchange chromatography due to rapid dissociation of hormone from SSB, post-labeling experiments yield binding activity eluting with 0.2 M KCl indicating that SSB is an acidic protein having a chromatography behavior similar to that of estrogen receptor. SSB binding was dramatically reduced by the chaotropic salt, NaSCN, whereas binding to the estrogen receptor was not disrupted. SSB is stabilized by sodium molybdate, a property which is characteristic of steroid receptors. Although the role of SSB in the chick oviduct is yet to be determined, an understanding of its properties is essential for accurate determinations of the estrogen receptor.