A fixed amount of 125I labelled 3α-Diol G competes with the 3α-Diol G to be measured present in the sample or in the calibrator for a fixed amount of antibody sites being immobilized to the wall of a polystyrene tube. Neither extraction nor chromatography are required because of the high specificity of the coated antibodies. After a 2 hours incubation at RT on a shaker, an aspiration step terminates the competition reaction. The tubes are then washed twice with 2 ml of Working Wash Solution and aspirated again. A calibration curve is plotted and the 3α-Diol G concentrations of the samples are determined by dose interpolation from the calibration curve.