• The Xg blood group antigen (a genetic marker of a region of the X chromosome at a considerable distance from protan/ deutan color blindness) was studied for linkage to bipolar manic-depressive illness. A multigenerational analytic method, taking variable penetrance into account, was used. In our series of six informative pedigrees, very close linkage could be definitively ruled out, and the likelihood of less tight linkage was consistently less than the likelihood of nonlinkage.