Abstract The erythrocyte defense system against cellular oxidants is complex and efficient. Free radicals generated in cell membranes, however, are relatively sequestered from the cell's antioxidant mechanisms. When a oxidant challenge exceeds the capacity of the erythrocyte's antioxidant system, membrane damage may occur, causing red cell destruction and hemolytic anemia. In this study, we present a method for monitoring radical reduction in eryhrocyte membranes, using fatty acid spin labels with cells are used. The electron paramagnetic resonance signals of the 5-doxylstearic acid spin labels in the intact cells are obtained as a function of time, at 37°C over a period of 2 h. The pseudo first-order rate constant for reduction of the spin label in normal adult intact cells under our experimental conditions is 4.3. ± 1.8 × 10 −3/min. The reproducibility and variability of the measurements are discussed. Since the measurements we describe reflect the extent of radical reductions occuring in cell membranes, we suggest that this method can be used to measure the ability to defend oxidants in membranes of erythrocytes with defective antioxidant systems. This method is particularly useful for measuring the modification of the antioxidant system toward radicals in membranes by drugs, chemicals, or environmental toxins.