Previous work with the antibody-blocking technique showed that the map of surface components for thymocytes prefixed with paraformaldehyde is the same as the map for unfixed thymocytes, with the following exception: after exposure to anti-TL or anti-Db, TL and H-2Db occupy adjacent positions on unfixed cells but not on fixed cells. This was interpreted as an indication that activation of particular components of the surface phenotype initiates ordered changes in the display of cell-surface molecules, approximation of TL and Db in this instance. These studies have now been extended to the H-Y component on the surface of male cells. On fixed male mouse thymocytes, H-Y lies adjacent to TL and relatively distant from H-2Db, H-2Kb, H-2Lb, Lyt-1.2, and Lyt-2.2. However, on unfixed male mouse thymocytes, similarly exposed to H-Y antibody, H-Y and H-2Db are adjacent. Presumably, this engagement of H-Y sites by H-Y antibody brings H-Y and H-2Db together. Evidence that this change in pattern may be physiologically relevant comes from the finding that testosterone, but not estradiol, caused the same selective approximation of H-Y and H-2Db.