We evaluated a new radioimmunoassay kit for unconjugated estriol in pregnancy plasma. The overall mean intra-assay precision (CV), as determined from replicate analyses of three plasma pools with different estriol concentrations, was 5%; the overall mean inter-assay precision was 7.7%. The assay system had acceptable linearity, with a correlation coefficient of 0.97 between results for 24 plasma samples assayed at 10 and 20 microL. Analytical recovery of estriol added to plasma to give three concentrations averaged 98.6%. Estriol values were generally higher with the kit than with our conventional charcoal-separation RIA method. The regression line equation was y = 1.11x + 1.0, the correlation coefficient 0.97. In plasma from 28 normal pregnant women, sampled serially during the last trimester, the mean unconjugated estriol concentration in plasma increased steadily from 29 nmol/L at 28 weeks of gestation to 42 nmol/L at 34 weeks, and then more rapidly to 93 nmol/L at term. This kit provides a rapid, technically simple, and reliable assay method, offering advantages to clinical laboratories with a high estriol workload.