NMR is a unique method for monitoring the structural and dynamic details of biomolecules. Furthermore, the non-destructive nature of NMR enables in-cell NMR, where proteins in their physiological context are monitored in live cells. Herein, we illustrate the first instance of protein NMR in live human cells, namely, the red blood cells (RBCs). After introducing 15N/13C-enriched α-synuclein into RBCs, we sought individual-specific NMR signals from α-synuclein. The environment inside RBCs was probed by the chemical shifts and line broadening of α-synuclein, as well as the pattern of post-translational modifications. To assess the diagnostic implications of NMR, we are comparing the NMR signals of α-synuclein in RBCs derived from patients with neurodegenerative disorders and the control group. We propose that NMR has the potential to be used for medical diagnosis by monitoring the inner environment of human RBCs through protein NMR.