Experimental results are reported for an unclad multimode optical fiber whose lateral surface is immersed in dilute fluorescein solution. This system provides a physical basis for fiber optic fluoroimmunoassay independent of immunochemical binding and kinetics. The excitation of fluorescein by evanescent wave interaction and subsequent detection of fluorescence from the optical fiber endface by the reciprocal process was studied. A systematic variation of optical launch conditions and cladding refractive index was completed. These results are compared with a semi-classical optical model which has been developed for this purpose. This predictive model yields results which for the first time are fully quantitative. Good agreement has been obtained between the experiments and the model. Additionally, sensitivity to 10 -9 molar solution has been obtained.