SUMMARY Subcellular localization of murine histocompatibility antigens (H-2) was studied in the ascitic forms of the SEWA sarcoma (H-28 specificity), YAC lymphoma (H-2a), and the TA3/Stockholm and TA3/Hauschka (H-2a) carcinoma sublines. Live cells were radioiodinated specifically at their cell surfaces by the lactoperoxidase method and the protein-bound radioactivity was used as a quantitative marker for the cell membrane content at each subcellular fraction. The H-2 antigens were assayed quantitatively by cytotoxic inhibition tests. The results showed that the H-2 antigens are almost exclusively localized in the cell membrane. The TA3/Hauschka cells are characterized by a low expression of H-2 antigen sites available for antibody binding on the cell surface of intact cells. Following homogenization, however, all of the antigenic sites became available, reaching about one-third of the level characteristic for the fully antigenic TA3/Stockholm line. The H-2 antigen of the TA3/Hauschka cells was entirely restricted to the cell membranes as well, and it must be therefore concluded that its reactivity is impaired by the steric arrangement in the living cells.