A method is described and evaluated for the determination of androstenedione (A) in peripheral venous plasma. The potential use of antisera to A-6-carboxymethyl thioether-bovine serum albumin and A-11α-succinyl-bovine serum albumin has been evaluated. The same plasma samples have been analysed before and after chromatography on a micro column of Sephadex LH-20. A dye, azobenzene is used to locate the fraction containing androstenedione. The results show that there is no significant difference in the values of apparent A using either antisera (overall mean 3.2 ng/ml plasma from men and 2.8 ng/ml from women). After chromatography these values are reduced by 62 and 42% respectively to 1.2 and 1.6 ng/100 ml. A new method using a mixture of ammonium and calcium sulphates is described for the separation of steroid bound to antibody. The precipitate is then resuspended and the amount of radioactivity determined, directly in the assay tube, by liquid scintillation counting. This process effects a 68 % reduction in the cost of the assay of each plasma sample.