We estimated in vitro membrane fluidity gradient in erythrocytes (RBC) from diabetic patients, using a fluorescent dye 1,6-diphenyl-1,3,5-hexatriene (DPH). The rate constant of DPH incorporation (k) into the membranes was determined by fitting experimental data to an exponential equation. Four important findings were made. First, membrane fluidity in the hydrocarbon region of RBC from diabetic patients is decreased compared with control cells (P<0.01). Second, the rate constant k of DPH incorporation into the membranes of RBC from diabetic patients was lower (P<0.01), which indicates an altered fluidity gradient in the membranes. Third, resorcylidene aminoguanidine (RAG) decreased significantly (P<0.001) the anisotropy values in RBC membranes from diabetic patients, which means that it apparently acted as a fluidizing agent. Lastly, no significant differences in the rate constants k were found between the control membranes (from RAG untreated RBC) and the membranes isolated from RAG pretreated blood from diabetic patients, as well as between the control membranes and those from RAG pretreated control blood. In conclusion, RAG affects lipid-protein interactions in RBC membranes, which results in membrane lipid bilayer fluidization and leads to the restoration of natural physiological membrane dynamic parameters in RBC from diabetic patients.