1. 1. Rapid loss in the characteristic maximum at 490 mμ occurs in solutions of fluorescein (uranin) below pH 7.0 with elimination of fluorescence at pH 2.0. A new absorption band appears in these solutions below pH 5.0 with a maximum at 430 mμ at pH 2.0. 2. 2. On the basis of samples of fluorescein amine II prepared in different laboratories, but having a melting point of 313 °, infrared absorption spectra have been obtained which serve as a standard for the preparation of this amine. 3. 3. The ultraviolet absorption spectrum has been shown of fluorescein amine isomer II in dioxane-acetone solution and of the change in spectrum which occurs after conversion by phosgene to fluorescein isocyanate. 4. 4. The ultraviolet absorption spectrum of the fluorescein nucleus in aqueous buffered solution, pH 7.6, showing a maximal band at 490 mμ and two minor bands at 280 and 325 mμ, has been obtained. The changes which occur in the intensity of these bands after conjugation to rabbit globulin have been shown. 5. 5. The emission spectra solutions of fluorescein and fluorescein-globulin conjugates have a longer wavelength than their absorption spectra with an optimum emission band at 550 mμ.