Five fluorimetric methods for the determination of oestrogens have been investigated and compared. The absorption spectra of the fluorescent compounds obtained in three have been studied, and the effect of water, toluene and alcohol have been investigated. The best method for high sensitivity is that of Bates & Cohen [1950a]; that of Finkelstein, Hestrin & Koch [1947], however, may be less influenced by any impurities present. Toluene, water and alcohol have a marked influence on the fluorescence. A mercury lamp may not be the most suitable source of light. A primary filter of maximum transmission at 450 mμ will give best sensitivity.