Radioimmunoassays were developed for the main plasma and urinary metabolites of PGF2alpha in the human, viz., 15-keto-13,14-dihydro-PGF2alpha and 5alpha, 7alpha-dihydroxy-11-ketotetranorprosta-1,16-dioic acid. The methods allowed assay of unextracted samples and had a sensitivity of 5 pg and 2 pg, respectively. The assays were evaluated by comparison with mass spectrometric data. Normal levels of these metabolites were estimated in a number of subjects. Levels in plasma and urine of the metabolites were followed after injection or during infusion of PGF2alpha and during administration of various antiinflammatory drugs.