This paper uses cytotoxic and proliferative T cell clones specific for H-Y and restricted by MHC molecules to type mice and humans inheriting incomplete portions of the Y chromosome. The data have allowed us to map the H-Y antigen gene Hya in mouse to a position closely linked with, but separable from, Tdy on the Sxr fragment and thus presumably to a position of the normal mouse Y chromosome near the centromere. The human H-Y gene maps between deletion intervals 4B and 7, separate from TDF which is on interval 1. We are currently testing cells from a number of additional patients who have inherited different portions of the Y chromosome to pinpoint the mapping more closely. It is of interest that in mouse a Y-linked gene controlling spermatogenesis (Spy) maps near Hya on the Sxr fragment: they could be the same or closely linked genes. In man, a gene controlling spermatogenesis maps to Yq and the data so far do not exclude that it could be coincident with the H-Y gene.