This report describes a radioimmunoassay method for the measurement of 20adihydro-progesterone (20a-P) in plasma. Utilizing a specific antiserum for 20a-P with o one-step Celite microcolumn chromatographic system, we have measured this steroid in small aliquots of plasma from both male and female subjects. In 10 men, the mean 20a-P level was 0.85 ng/ml. In six premenopausal women studied during one complete menstrual cycle, the mean levels during the follicular and luteal phases were respectively 0.63 ng/ml and 3.7 ng/ml. These values are within the range of levels reported by one group of investigators but higher than those reported by another group.