A radioimmunoassay (RIA) for prostaglandin E₂ (PGE₂) based on the use of commercial PGF_2α antiserum is described. PGE was extracted from human urine and separated by silicic acid column chromatography, then PGE was converted to PGF by the use of sodium borohydride (NaBH₄), and the formed PGF_2α was determined radioimmunologically with commercial PGF_2α antiserum. There was a good correlation (y=0.841x+0.518, r=0.912) between added standard PGE₂ and measured PGE₂ levels in human urine. Moreover, a good correlation (y=1.46x+0.569, r=0.952) was also found between the PGE₂ levels in the urine obtained by means of the bioassay previously reported and those obtained by the proposed method. Therefore, this method has high accuracy and good reproducibility. Furthermore, it is also an advantage that radioimmunological assays of PGF_2α and PGE₂ in biological samples can be performed separately with only one antiserum by this method.