Abstract Erythrocytes from Duchenne dystrophy patients lysed more readily than red cells from age-matched normal boys when lysophosphatidylcholine (LPC) concentrations that caused 50% lysis were compared. Erythrocytes from myotonic dystrophy patients appeared to be more resistant than cells from age-matched normal adults at certain medium LPC concentrations. Erythrocytes from patients with Huntington's disease showed no significant differences from erythrocytes of normal adults. Thus, the manner in which erythrocytes respond to LPC may reflect the putative membrane alterations in these diseases. Inhibition of LPC-induced lysis by 0.1 mM dipyridamole was observed in all groups. Since this agent did not inhibit LPC lysis at 0 °C, its action at 37 °C could be related to activation of a membrane enzyme. On the other hand, dipyridamole decreased osmotic fragility at 0 °C and 37 °C indicating that a physical change in membrane structure may be the primary alteration produced by this agent.