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In a recent population study, we observed a striking deficit of G6PD heterozygotes among Southern African Negroid females. This finding was interpreted tentatively as evidence for a small number of hematopoetic stem cells in man. In a follow-up study we examined peripheral blood and cord blood in 547 mothers and in their newborn offspring. In mothers and sons, the frequencies of the G6PD alleles are apparently quite different. When the allele frequencies determined in sons are used for calculation of the expected phenotype frequencies in mothers and daughters, there is a large deficit of maternal G6PD AB phenotypes, and an equivalent surplus of G6PD homozygotes. However, no relevant heterozygote deficit is observed in newborn daughters. This discrepancy may be explained by the assumption that in peripheral blood of heterozygotes carrying the GdA- allele, G6PD-deficient cells progressively become eliminated during development from birth to adulthood. In other words, the large heterozygote deficit observed in adult females may be due to somatic selection rather than to a small pool of hematopoetic cells at the time of X differentiation.