Three alloantigens encoded by Ly-6-1inked genes are defined by monoclonal antibodies. The Ly-27.2 antigen is defined by antibody . The strain distribution pattern of these antibodies is the same and identical with Ly-6.2. However the tissue distribution of these antigens is unique and distinguishes these antigens from the Ly-6.2 antigen or any known antigen encoded by Ly-6-linked genes. Ly-27.2 is present on all thymocytes, T cells, and B cells but is absent from bone marrow cells, whereas Ly-28.2 is absent from most thymocytes and is present on a subpopulation of T ceils and B cells but is found on 60-70% of bone marrow cells. No recombination between the Ly-6/Ly-2 7/Ly-28 loci was found in linkage studies using 41 recombinant inbred strains and 57 backcross mice and indicates very close linkage of these genes. In addition, close linkage to 24 minor histocompatibility genes was excluded using the Bailey HW bilineal congenic mice. The data presented indicate that either the Ly-6 complex is composed of a family of tightly linked genes or the antigens are the products of a single gene that undergoes extensive modification during differentiation. * The cell lines secreting Ly-27.2-specific or Ly-28.2-specific antibodies were derived from a fusion using RIII spleen cells (immunized with LP.RIII lymphoid cells). The anti-Ly-28.2 secreting cell line 5095-16.6 was derived by fusing BALB/c spleen cells immunized with B 10.D2 thioglycolate-induced PEC. Cytotoxicity performed using rabbit complement, titer = 50~o lysis.