The data from this investigation show that the anti-VS sera studied cannot be separated into anti-hrv (anti-V) and anti-hrH specificities. The antibody specificity is singular and is directed against an antigenic determinant present on VS positive red blood cells. That some anti-VS sera appear to have a separable specificity may be due to the incomplete absorption of those antisera. It was shown that V-, VS+ red blood cells adsorb more anti-VS than do those that are V+, VS+. Several absorption of anti-VS serum with V+, VS+ red blood cells may remove enough of the antibody that the absorbed serum will no longer (visibly) react with V+, VS+ red blood cells, though the same absorbed serum will react with V-, VS+ red blood cells. That this is not a separable specificity can be demonstrated by subsequent absorption to exhaustion of the same serum with V+, VS+ red blood cells. Testing of the original Hernandez serum comfirmed that it defines the antigenic specificity, hrH. The relationship of hrH to VS may be similar to the relationship of the Rh0 (D) antigen to the antigens of the Rh0 (D) mosaic.