A simple radioimmunoassay for serum reverse [r] T3 on small reusable Sephadex G 25 fine columns was developed. The assay was unaffected by serum volumes of up to 100 .mu.l. The recovery of rT3 added to serum was in the mean 98.3 .+-. SEM [standard error of the mean] 3.2% and the coefficient of variation within and between assay determinations 5.4% and 7.5%, respectively. The detection limit was 2.2 pg rT3/column. Serum reverse T3 in 58 euthyroid controls was in mean 48 .+-. SD 9 ng/100 ml, and was positively correlated to age (P < 0.001). No overlap was found between the control group, and the hypo- and hyperthyroid group, respectively. Three patients with T3-toxicosis had a normal serum rT3.