The membrane effects of ticlopidine on the erythrocyte membrane were explored by the spin label method at the proteic and phospholipidic levels. This spectroscopic study was completed by polyacrylamide gel electrophoresis of proteins, measurement of the protection against haemolysis and observation of the erythrocyte shape changes induced by the drug. Two types of effects have been observed. At concentrations higher than 5 x 10(-4) M, ticlopidine is a weak denaturating agent of the membrane proteins. At concentrations of pharmacological interest, the main effect of the drug is a protection against hypotonic haemolysis, and an increase in the fluidity of the membrane phospholipidic core. This last result could explain in part the interesting pharmacological effect of ticlopidine on various circulatory troubles.