Lipid-soluble spin labels were used to probe the fluidity of membrane lipids in erythrocyte membranes from patients with Duchenne muscular dystrophy. A greater mobility of spin label motion was noticed at the surface of the Duchenne erythrocyte membranes in comparison with the similary labeled normal human erythrocytes. Changes in the protein conformation and/or protein-lipid organization, as a consequence of an altered membrane lipid fluidity, have been demonstrated by the use of a sulfhydryl group specific protein spin label. Strongly immobilized sulfhydryl groups appear to be located both at the membrane surface and deep within the lipid bilayer. The ratio of the spectral amplitude of the spin label attached to weakly immobilized sulfhydryl groups to that of strongly immobilized SH-groups is significantly greater in Duchenne membranes, compared to those of controls. These studies suggest alterations in lipid-protein organization at the surface of Duchenne erythrocytes, indicating that Duchenne muscular dystrophy may be a disease resulting from a membrane abnormality at the molecular level.