A modified procedure is developed for estimation of superoxide dismutase activity using inhibition of reduction of tetranitrotetrazolium blue (NTB) by the superoxide radicals, produced in the system xanthine-xanthnie oxidase. In the modified procedure initial concentration of NTB was decreased 2-fold, which enabled to increase 1.7-fold the sensitivity of the method. The value of the superoxide dismutase activity was unaltered in mice liver cytosol if the initial rate of NTB reduction varied from 0.01 to 0.02 min-1. The values of inhibition of the enzymatic activity were maintained up to 0.032 min-1 in the systems with purified superoxide dismutase preparations. Separation of low molecular metabolites did not alter these ranges of the constant inhibition values under the conditions of various initial rates of NTB reduction in supernatants.