Duchenne erythrocytes showed increased osmotic fragility as compared to controls (p less than 0.01), but individual values overlapped with controls, and only half of the Duchenne erythrocyte values were abnormal. When the effect of the smaller mean corpuscular volume of Duchenne erythrocytes was taken into account, there was no significant difference from controls in membrane deformability, as determined by microsieving or flow channel measurements. The increased osmotic fragility suggests minor changes in erythrocyte membrane properties in Duchenne muscular dystrophy.