The use of dexamethasone (DXM) as a generally applicable blocking agent in steroid RIA was investigated. Precision of estradiol (E2) and testosterone RIA systems was much improved after treatment with DXM. Addition of DXM reformed the standard curves made in hormone free serum (HFS) matrix. Accuracy as percent recoveries of E2spiked in HFS were 115.7 ± 71.2 and 107.4 ± 14 for direct and after addition of DXM, respectively. For T assay these values were 54 ± 23.1 and 103.3 ± 9.5. Direct E2and T RIA using DXM in control serum samples of low, medium, and high levels gave comparable values to solvent extraction. DXM was found to be a potent blocking agent in steroid RIA.