The permittivity ( epsilon ') of blood of four different haemoglobin concentrations was measured at room temperature at a frequency near 2 GHz. The permittivity of the plasma was also determined. The variation of permittivity with concentration was shown to exhibit a good straight line relationship corresponding to a change in epsilon ' of 0.8 (about 1.3%) for a change of haemoglobin content of 1g Hb per 100 ml of blood.