Abstract: A system for suppressing normal serum fluorescence in fluorescence immunoassay, in which in one embodiment, the immunoassay fluorescently tagged reaction product in a single sample is irradiated at a first wavelength which will excite fluorescence from the tagged product and then at a different wavelength which is not substantially in an absorption band of the fluorescent tag, the respective fluorescent emissions being detected and converted to electrical signals. The difference between the signals, adjusted for variation in serum fluorescence at the two wavelengths, is a function of the antigen titer. In an alternative form, the sample is irradiated at the first wavelength for a predetermined period of time commencing very shortly after the emission reaction is started, and the resulting signal from the detector is differentiated. In both instances, output signal due to serum fluorescence per se is suppressed.