We found that none of twenty natural amino acids quenches the fluorescence of anionic PTP, but phenylalanine, tryptophan, and tyrosine do quench obviously the fluorescence of cationic PFP. The overlaps between the Q-band absorption spectrum of myoglobin, cytochrome c, and horseradish peroxidase and the fluorescence emission spectrum of PTP indicate that the fluorescence quenching of PTP by hemeproteins is via fluorescence resonance energy transfer, not via photoinduced electron transfer.