Abstract: A direct radioimmunoassay of serum cortisol was developed using a highly specific antibody and 8-anilino-l-naphthalene sulfonic acid (ANS) as a blocking agent to inhibit the binding of cortisol to serum proteins. ~(125)I-cortisol-3-oxime-histamine was used as the labelled antigen. The intra-assay coefficient of variation was 4.5 and inter-assay coefficient of variation 6.5%. Recovery: 102% and sensitivity: 50 pg. The values found in normal human serum were comparable with those reported using other methods.