An obligate carrier of the Wiskott-Aldrich syndrome (WAS) who was also heterozygous for the A and B types of X-linked glucose-6-phosphate dehydrogenase was found. With her it became possible to determine whether allelic exclusion occurs in particular cell-types of the WAS carrier. If so, the remaining cells of a particular cell-type would express only the normal X chromosome and only one glucose-6-phosphate dehydrogenase isoenzyme would be demonstrable. This carrier had only the B isoenzyme of glucose-6-phosphate dehydrogenase in platelets and thymus-derived T lymphocytes, although both isoenzymes A and B were present in erythrocytes and neutrophils. These findings suggest that selection against the WAS gene occurs in platelets and thymus-derived T lymphocytes and that the defects associated with WAS expressed in these cell-types may be implicated in the genesis of the Wiskott-Aldrich syndrome.