New rare earth chelate fluorescent probes with decay time constants much longer than the background decay time-constant of typical. organic materials offer the potential of a great improvement in sensitivity of time-resolved fluorimetry. However, if they are to rival radioimmunoassay methods the sensitivity requirement becomes so great that overload recovery of the fluorescence detector is a major problem. Extensive past work on this problem is largely inapplicable to the very low speed and level requirements of this situation. Consequently initial effort is being applied to determining the optimum fluorescence detection device and ancillary signal overload limiting circuitry. Problems related to the fluorescent probe at extremely low sample concentrations will be studied in collaboration with CDC, which is interested in this approach as another tool for AIDS research. Finally an evaluation of the details of excitation, optical filtering, and digital signal processing will be worked out and optimized.