The fluorescence intensity of the dye Fluorol(R) 7GA in in vitro caries lesions was measured through a strip of plastic either 2 or 1.5 mm in thickness. The light scattering in the plastic strips was comparable with light scattering in sound enamel. This method was chosen as a model for fluorescent dye measurement at approximal sites, where the fibreoptic probe cannot be placed directly on the lesion, so that measurements have to be made through a thin layer of sound enamel. It appeared that the fluorescent dye can be measured through a layer of scattering material and that a linear correlation exists between fluorescence intensity and calcium loss as measured by longitudinal microradiography. The correlation coefficient for the 2-mm plastic strip was r = 0.76. With the 1.5-mm plastic strip the fluorescence intensity recordings were about twice as high, and the correlation coefficient was r = 0.81. From these measurements a proposal for an application procedure emerged for the use in a clinical situation. Also the transmission of light at 550 and 650 nm, at which wavelengths the fluorescent dye does not absorb, through plastic strips of varying thickness was measured. It is shown that by measurement of transmission of a non-adsorbing wavelength, the fluorescence intensity measured through a layer of scattering material can be corrected for this layer.