The quantity of steroid hormone receptors in human mammary carcinomas is of importance for predicting clinical response to hormone modes of therapy. There have been several reports describing an increase in the yield of cytosolic receptors when sodium molybdate is included in the tumor cytosol. Since the quantitative units for receptors are fmol hormone bound per mg cytosolic protein, we examined whether this oxyanion had an interfering or enhancing effect on two methods of protein measurements: those of Bio-Rad and of Lowry. Our results showed that sodium molybdate, even when present in a 5 mM concentration in the protein solution, interferes with the color development by both reagents. This effect of molybdate is consistent, reproducible and statistically significant. We present the data to caution the investigators to be aware of this phenomenon while interpreting their own data on the molybdate effect on steroid receptors.