We have developed a new technique, fluorescence differential path length spectroscopy (FDPS), that enables the quantitative investigation of fluorophores in turbid media. FDPS measurements are made with the same probe geometry as differential path length spectroscopy (DPS) measurements. Phantom measurements are performed for two fiber diameters (400 microm and 800 microm) and for a wide range of optical properties (mu(s)': 0 to 10 mm(-1); mu(a): 0 to 2 mm(-1)) to investigate the influence of the optical properties on the measured differential fluorescence signal. The differential fluorescence signal varies by a factor of 1.4 and 2.2 over the biologically relevant scattering range (0.5 to 5 mm(-1)) for a given fluorophore concentration for 400 microm and 800 microm fibers, respectively. The differential fluorescence signal is attenuated due to absorption at the excitation wavelength following Lambert-Beer's law with a path length equal to the differential path length.