Resazurin is a cell permeable redox indicator that can be used to monitor viable cell number with protocols similar to those utilizing the tetrazolium compounds (26). Resazurin can be dissolved in physiological buffersbuffers buffersbuffers(resulting in a deep blue colored solution) and added directly to cells in culture in a homogeneous format. Viable cells with active metabolism can reduce resazurin into the resorufinresorufin resorufinresorufinresorufin product which is pink and fluorescent fluorescentfluorescent fluorescent fluorescentfluorescent(Figure 7).