Previous data on in vitro culture of Plasmodium falciparum malaria demonstrated that red cell glucose-6-phosphate dehydrogenase deficiency (G6PD-) inhibited parasite growth in deficient hemizygous males. This study investigated the effect of heterozygosity for G6PD- on parasite growth. Blood was obtained from 8 female Sardinian G6PD- heterozygotes with G6PD normal cells ranging from 13% to 60%. For comparison, blood from a G6PD- hemizygous male, containing 100% deficient red cells, was mixed in different proportions with compatible normal blood. In both experiments, parasite growth was inhibited by the presence of deficient cells. In both cases, it was found that the inhibition could be explained by a simple dilution of normal cells by G6PD- cells. Thus, the typical female heterozygote is also protected to a significant extent. When considering the "malaria hypothesis" as it relates to G6PD, protection of the female heterozygote as well as the male hemizygote must be taken into account.