The activity characteristics of membrane acetylcholinesterase from red blood cells of diabetic patients are very different from those of healthy donors: the limiting enzyme reaction rate is 17.2 +/- 0.8 mumol acetylthiocholine per ml packed cells per min compared with 13.1 +/- 0.8 mumol for control cells. This Michaelis constants for substrate are the same: 0.061 +/- 0.007 mM for diabetic and 0.061 +/- 0.004 mM for control cells. Cell exposure to oxidative agent (t-butyl hydroperoxide) significantly changes the enzyme activity parameters. The limiting enzyme reaction rate increases but the affinity for the substrate decreases at lower oxidant concentrations (up to 0.1 mM for the "diabetic" erythrocytes and up to 0.4 mM for the control ones). At higher oxidant concentrations both the limiting reaction rate and the Michaelis constant decrease. The susceptibility of erythrocyte membranes of diabetic patients to oxidative stress is much higher in comparison with control erythrocyte membranes.