Caged peptides, whose activities are masked by the introduction of photocleavable groups, have recently been recognized as a useful tool to elucidate various biological phenomena with a high spatial and temporal resolution, even in living cells. AIST has developed two types of caged peptides; side-chain caged peptides and backbone-caged peptides. The technique was applied to sperm activating peptide (speract). The backbone-caged speract showed superior features to those of the side-chain caged speract: greater caging ability and faster photolysis rate. The caging strategy described can be used as a general procedure to cage any biologically active peptide.