SUMMARY: 1. The estimation of oestrogens by means of their fluorescence in phosphoric acid has been investigated in some detail. 2. A highly sensitive fluorimeter is described; with suitable filters it was possible to measure accurately 0·01 μg of oestradiol, 0·02 μg of oestrone and 0·05 μg of oestriol. 3. Several light sources and filter combinations have been studied. The most satisfactory light source is a bright tungsten lamp, and for maximal sensitivity a primary filter consisting of a Wratten 49 and an Ilford 809 together with a secondary filter combination of Ilford 404 and 110 are used. 4. Interference with fluorescence is discussed. This arises from insufficiently pure reagents and incomplete purification of the oestrogens when extracted from body fluids. Two tests for the presence of quenching are suggested; they can also be applied quantitatively to correct for such interference, although their validity is not yet proved.