A high frequency of nonhemolytic hereditary ovalocytosis in Malayan aborigines is thought to result from reduced susceptibility of affected individuals to malaria. Indeed, Kidson et al. recently showed that ovalocytes from Melanesians in Papua New Guinea are resistant to infection in culture by the malarial parasite Plasmodium falciparum. In order to determine if protection against parasitic invasion in these ovalocytes might be the result of some altered membrane material property in these unusual cells, we measured their membrane and cellular deformability characteristics using an ektacytometer . Ovalocytic red cells were found to be much less deformable in comparison to normal discoid red cells. Similar measurements on isolated membrane preparations revealed a marked reduction in ovalocytic membrane deformability. To produce equal deformation of ovalocytic and normal membranes, ovalocytes required an 8-10-fold increase in applied shear stress, indicating that their membrane was capable of deforming under sufficient stress. To test the possibility that this increased membrane rigidity might confer resistance to parasitic invasion, we performed an in vitro invasion assay using Plasmodium falciparum merozoites and Malayan ovalocytes of varying deformability from seven different donors. The level of infection of the ovalocytes ranged from 1% to 35% of that in control cells, and the extent of inhibition appeared to be closely related to the reduction in membrane deformability. Moreover, we were able to induce similar resistance to parasitic invasion in nonovalocytic normal red cells by increasing their membrane rigidity with graded exposure to a protein crosslinking agent. Our findings suggest that resistance to parasite invasion of Malayan ovalocytes is the result of a genetic mutation that causes increased membrane rigidity.