The sulfhydryl group specific spin labels have been used to study the membrane proteins in the bovine and human erythrocyte membranes treated with ozone. The ratio hw/hs, determined from the respective peak amplitudes of the resulting EPR spectra of 3-maleimido-1-oxyl-2,2,5,5-tetramethylpyrrolidine bound to the ozone treated membranes, was shown to be elevated following an increased ozone concentration. The ratio of the central to high-field peak amplitude h0/h-1, a measure of the rotational correlation time, determined from EPR spectra of ozonized membranes covalently labeled with 4-(iodoacetamido)-1-oxyl-2,2,5, 5-tetramethylpyrrolidine was shown to be diminished. The changes in the erythrocyte membranes treated with ozone, observed for both spin labels, indicate a dose dependent increase in the mobility of membrane proteins.