Rhodamine B thiohydrazide (RBS) was firstly employed as turn-on fluorescent probe for hypochlorite in aqueous solution and living cells. It exhibits a stable response to hypochlorite from 1.0×10-6to 1.0×10-5M with a detection limit of 3.3×10-7M. The response of this probe to hypochlorite is fast and highly selective compared with other reactive oxygen species (such as.OH,1O2, H2O2) and other common anions (such as X-, ClO2-, ClO4-, NO3-, NO2-, OH-, Ac-, CO32-, SO42-).