Recently developed techniques for the direct analysis of DNA have made possible the determination of patterns of cellular X-chromosome inactivation. These techniques provide a potential method for carrier detection for several X-linked human disorders in which obligate carriers show nonrandom X inactivation. By using restriction fragment length polymorphic (RFLP) gene-specific probes in conjunction with methylation-sensitive enzymes, we have characterized the patterns of X-chromosome inactivation in cell subsets from females belonging to 10 kindreds segregating for the X-linked immune deficiency disorder Wiskott-Aldrich syndrome (WAS). We show that selective inactivation of the X chromosome distinguishes obligate WAS carriers from noncarrier females and constitutes a valuable marker of the WAS carrier state. Selective inactivation phenomena were observed in the monocytes and T and B lymphocytes of obligate carriers, implying that the WAS gene defect is expressed in each of these cellular lineages. In conjunction with the use of linked DNA markers, RFLP-methylation analysis should render carrier detection feasible for the majority of females from WAS families. The results of such analyses also provide an initial step toward identifying the cellular level and molecular basis for WAS.