The cytoplasmic estrogen receptor (ER<sub>C</sub>) and progesterone receptor (PR<sub>C</sub>) in mammary tumors have been recognized as useful biochemical markers for predicting the objective response of patients with advanced breast cancers to endocrine therapy. These proteins are also useful in the prognosis of gynecologic carcinoma. This report presents data showing the effect of sodium molybdate in the stabilization of estrogen and progesterone receptors. In rabbit uterine tissue, molybdate (20 m M) increased the binding of progesterone and estrogen to the receptors in several ways: (a) the apparent loss of detectable receptors during lengthy sucrose gradient analysis and at elevated temperature (30°C) was reduced; (b) the instability of receptors due to storage at -70°C was lessened, and (c) the conversion of the 7 SPR<sub>C</sub> to the 3.5 S form was minimized. Similarly, molybdate caused a qualitative and/or a statistically significant quantitative difference in receptor values for some human gynecologic tumors presented herein; the molybdate-associated changes vary with tumor specimen. Of the 8 tumors for which receptor values in the presence of molybdate (M+) and its absence (M-) can be compared, detectable ER<sub>C</sub> of 6 and PR<sub>C</sub> of 7 tumors increased with molybdate, and ER<sub>C</sub> of 2 and PR<sub>C</sub> of 1 tumor showed no change. In addition to the increase in receptor values, a concomitant shift of the 3–4 S molecules to the 7–85 moieties was noted for some tumors (1 of 6 for ER<sub>C</sub> and 3 of 7 for PR<sub>C</sub>). In 2 receptor-poor tumor samples, ER<sub>C</sub> was only detected in M+ cytosols. These results show that molybdate is effective in reducing receptor degradation and stabilizes the 7–85’ molecules from converting to 4 S moieties. The addition of molybdate may be helpful for better quantitation of steroid receptors in clinical specimens.