How to measure the intra-axonal diffusion coefficient in the normal human brain? This contribution reports on such a measurement performed with minimum model assumptions. The result is about $$$2\,\rm\mu m^2/ms$$$, which constructively limits the scope of acceptable biophysical models for interpretation of diffusion MRI in terms of tissue microstructure. A side finding is the width of axonal orientation distribution in the most coherent fiber bundles, which is estimated as $$$19^\circ$$$ for the standard deviation. The core of the method is the suppression of signal from water that can move in the plane transverse to the principal fiber direction.