The physical and dynamic properties of intact erythrocyte membranes of two groups, diabetic patients treated by diet alone (27 women and 41 men, 40-75 years old) and healthy subjects (controls) (25 women and 35 men, 35-75 years old) were investigated by electron spin resonance (ESR) spectroscopy. 5-doxyl stearic acid (5-DSA) and 4-maleimido-2,2,6,6-tetramethyl-piperidine-1-oxyl (MAL-6) are employed as spin labels throughout the experiments. While 5-DSA monitors the membrane surface region and gives information about the order of lipids in this region through the order parameters S', MAL-6 binds thiol groups S-H to the membrane protein and gives information about possible protein conformational changes. Order parameters were calculated from room temperature spectra recorded for intact erythrocytes labeled with 5-DSA and the values of 0.682 +/- 0.004 and 0.666 +/- 0,004 were obtained for diabetic patients and controls, respectively. Variations of the h((-1)), h((o)), h((+1)) and 2A(parallel to) parameters in the temperature range of 10-50 degrees C were also studied and fairly different behaviors were observed for both groups, especially for the h((-1)) parameter. Line intensities W and S characterizing the concentrations of weakly and strongly immobilized MAL-6 species bound to S-H groups of intact erythrocyte membrane proteins were used to calculate the W/S ratios. They were found to be 34.1 +/- 1.9 and 30.5 +/- 2.4 for diabetic patients and controls, respectively. While line intensities T, those of the M-1 = -1 high-field resonance line associated with the weakly immobilized S-H sites, of both groups exhibit a very similar microwave saturation behavior in the range of 0.5-20 mW, line intensities W of both groups saturated differently. Variations of line intensities S, W and T with temperature were also investigated in the temperature range of 10-50 degrees C and characteristic differences were observed to exist between two groups implying a less fluid erythrocyte membrane for diabetic patients. Increases in the order parameter S' and in the W/S ratio are considered as major factors attributable to the alteration in the physical and dynamic properties of erythrocyte membrane in the diabetic state.