An optical method is presented that allows the measurement of the triplet lifetime of a fluorescent molecule. This is a characteristic specific to each fluorophore. Based on differences in triplet lifetimes of two fluorescent species (autofluorescence versus label), this novel approach measures relative quantities of a transmembrane receptor and associated fluorescently labeled ligand during its recycling in living cells. Similarly to fluorescence-lifetime based methods, our approach is almost insensitive to photobleaching. A simple theory for unmixing two known triplet lifetimes is presented along with validation of the method by measurements of transferrin recycling in a model system based on chinese hamster ovarian cells (CHO). Transferrin is the delivery carrier for Fe3+ to the cell.