We propose to measure the diffusion of water in human red blood cells on the high resolution spectrometer IRIS with quasi-elastic neutron scattering. Diffusion of water in cells consist of a slow translational component and a fast rotational component. Due to the high energy resolution of IRIS we would be able to measure only the translational diffusion, the fast rotational diffusion would appear as flat background. Our aim is to show that there occurs a transition of translational water diffusion at human body temperature. To focus on water dynamics we intend to perform a difference experiment of red blood cells in H2O and D2O. As the incoherent scattering cross section of H is much bigger then D, we shall extract the dynamics of H2O by subtracting the intensity of scattered neutrons of the sample in D2O from that of the sample in H2O.