A new radioimmunoassay for urinary prostaglandins E and F alpha is described which allows the extraction and chromatographic separation of urinary prostaglandins with an overall recovery of 80.5% and 89.3% for prostaglandins E2 and F2 alpha respectively. The antibodies raised in the rabbit could not distinguish prostaglandins of series 1 and 2 but were found highly specific towards their metabolites and other primary prostaglandins. The sensitivity of the assay developed was at 2.2 pg for prostaglandin E and 2pg for prostaglandin F alpha. This assay was applied to normal adults of both sexes; it was shown that the daily urinary excretion of prostaglandins F alpha and E was greater in men than in women. In children aged from 1 to 114 months, the urinary excretion of prostaglandins increased with age. This increase was more marked during the first two years of life and was significantly related to age.