The characterisation of erythrocyte stiffness and its heterogeneity in a blood sample is a key parameter for describing erythrocyte pathologies. I will present a method for probing the intracellular rheology of red blood cells based on molecular rotors, which are fluorescent probes sensitive to viscosity in viscous fluids. I will illustrate the interest of this approach in the context of red cell pathologies such as sickle cell anaemia. A. Briole, T. Podgorski & B. Abou, Molecular rotors as intracellular probes of red blood cells rigidity, Soft Matter 17, 4525 (2021). A. Briole & B. Abou, Molecular rotors in haemoglobin and bovine serum albumin proteins, Journal of Royal Society Interface 19: 20220709 (2022); doi.org/10.1098/rsif.2022.0709.