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Filtrability of erythrocytes obtained from uncontrolled Type 1 (insulin-dependent) diabetic patients is abnormal, but is corrected by insulin added in vivo or in vitro. As erythrocyte filtrability depends on several determinants, we chose to study a membrane property of erythrocytes from diabetic subjects. Membrane fluidity was studied by fluorescence polarization using a lipophilic probe, the diphenyl-hexatriene and the Coulter Epics V together with a laser Spectra-physics 2000. Fluorescence polarization values obtained for 31 normal subjects (0.253 +/- 0.043 SD) and 31 uncontrolled Type 1 diabetic patients (0.231 +/- 0.043 SD) were significantly different (p less than 0.01). Insulin (2.5.10(-9) mol/l) added in vitro increased the fluorescence polarization values of red cell membranes from diabetic patients (without insulin, fluorescence polarization values = 0.210 +/- 0.032 SD; with insulin, fluorescence polarization values = 0.253 +/- 0.024 SD, p less than 0.001, n = 15), but had no effect on normal membranes (without insulin fluorescence polarization values = 0.255 +/- 0.037 SD, with insulin, fluorescence polarization values = 0.251 +/- 0.026 SD; n = 12). Given a relationship between the lipid bilayer and membrane cytoskeleton proteins, this insulin-correctable abnormality of erythrocyte membrane fluidity may be an important determinant of the rheological behaviour of erythrocytes from diabetic patients.