Cells from eight of ten patients with gonadal dysgenesis and an isochromosome for the long arm of X, (i(Xq)), have been found to be H-Y antigen-positive, using an assay that employs rat antiserum and Raji cells. In addition, two patients with del(Xq) were also found to be H-Y antigen-positive, whereas four patients in whom only a 45,X line was detected were H-Y antigen-negative. These findings suggest that the X chromosome plays a role in the expression of H-Y antigen in the absence of a Y chromosome. Since our patients with i(Xq) show no evidence of testicular differentiation, it is clear that there is not enough H-Y antigen on these patients' cells to direct the development of a testis. These findings are consistent with the view that the normal functioning of genes on the X and the Y chromosomes is necessary for testicular organogenesis to occur.