Abstract We describe a rapid, simple assay for placental estriol in urine, involving a small sample volume (250 microL) and, correspondingly, a small amount of enzyme reagent. After hydrolysis, estriol is adsorbed from urine onto graphitized carbon black (Carbopack B). After some washing steps, estriol is desorbed with a small volume of a suitable mobile phase. This single-step purification technique is rapid (about 15 min), with minimal sample manipulation. Analytical recoveries for estriol-supplemented urine ranged from 95.9 to 101.0%. Ten replicate analyses of urines containing typical concentrations of estriol gave CVs of 3.3, 2.6, and 2.4% for low, medium, and high concentrations, respectively. The specificity of our extraction technique was good, as assessed by treating 35 urine samples and quantifying estriol by gas chromatography with packed and capillary columns and by liquid chromatography with ultraviolet and fluorometric detection.