An improved competitive protein-binding radioassay for total plasma cortisol, which permits rapid analysis of many samples, is described. Plasma [human], 0.1 ml, is directly applied to a Whatman 3mm filter-paper strip, which is extracted once with dichloromethane. The extract is evaporated and the residue is assayed directly by a micro-method. Comparison with the ethanol extraction method yielded excellent agreement (correlation coefficient = 0.988). One hundred samples, in duplicate, can easily be assayed at a time.