rT3 RIA standard curves were generated using rT3 standards from various sources. The rT3 antiserum and the method of free and bound hormone separation used were the same in all assays. The slopes of rT3 RIA curves differed considerably; standard curves generated with Henning (Berlin) rT3 standard had the steepest slope. Because of the differences in slopes of standard curves, bound to free estimates, when extrapolated from standard curves generated with various rT3 standards, resulted in 2- to 3-fold differences in rT3 assay values in the same sera. Circulating rT3 concentration in human sera was 20 ng/100 ml when Henning rT3 standard was used in the assay, whereas with Jorgensen's rT3 standard, a value of 42 ng/100 ml was noted. As the rT3 antiserum and assay conditions used were identical in all comparisons (except rT3 standards), the 2- to 3-fold differences in rT3 values noted in the same sera suggested variability in potency of rT3 standards. The differences in rT3 values in the same sera (resulting from extrapolation from standard curves using different rT3 standards) were of the same magnitude as that noted in normal human sera in various studies, suggesting that the reported rT3 variations in sera of euthyroid individuals may also be due to differences in rT3 standards employed in the assay.