Radioimmunoassay technique for measuring 5alpha, 7alpha-dihydroxy-11-keto-tetranorprosta-1, 16-dioic acid, the main urinary metabolite of PGF1alpha and PGF2alpha (PGF2alpha-MUM), was further improved. It was postulated based on some experimental data that the PGF2alpha-MUM exists in the urine mostly as dioic acid form, not as delta-lactone form the daily excretion of PGF2alpha-MUM in men ranged from 14.43 mug to 36.14 mug and in women from 5.21 mug to 14.25 mug.