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Fluorescence quenching in chlorophyll-a induced by radiationless intermolecular singlet-triplet energy transfer is reported. A two-laser excitation procedure is employed to study this quenching. The method complements fluorescence spectroscopy for measurement of fast intramolecular relaxation processes. The observation of singlet-triplet Annihilation suggests the possibility of a central role of a higher triplet state of chlorophyll-a in photosynthesis.