Normal human venous blood was studied by electron paramagnetic resonance (EPR) spectroscopy at -196 °C. The EPR signal of free radicals in frozen blood is shown to have the same radiospectroscopic parameters and properties as the signal of the globin based free radical, ·Hb(Fe(IV)=O), formed in the reaction of purified methemoglobin (metHb) with H2O2 and therefore has been assigned as such. The globin-based radicals and metHb exhibited significant variation (fluctuations) in different frozen samples taken from the same liquid blood sample. In any given sample a high concentration of free radicals was associated with a low concentration of metHb and vice versa, i.e. the fluctuations were always of opposite sense. No such fluctuations were observed in the concentration of two other paramagnetic components of blood, transferrin and ceruloplasmin. The time course of free radical formation and decay upon the addition of H2O2 to purified metHb was studied at three different molar ratios H2O2/metHb. This kinetic study together with the results of an annealing experiment allow us to propose a mechanism for the formation and decay of the globin-based radical in blood. Within this mechanism, the source of H2O2 in blood is considered to he dismutation of O2· radicals produced via autoxidation of Hb. We postulate that the dismutation is intensified on the phase separation surfaces during cooling and freezing of a blood sample. The fluctuations are explained within this hypothesis.