SummaryThe immune response of SJL/J (H‐25) mice to syngeneic reticulum cell sarcoma (RCS) tumour cells was investigated. The nature of tumour‐associated antigens was examined by immunological and biochemical techniques. Reticulum cell sarcomas of different origins (spontaneous, transplantable and cultured cell lines) stimulate a strong syngeneic response measured by [3H]‐thymidine incorporation. However, only the cell line were able to stimulate a syngeneic cell‐mediated cytotoxic response. Further analysis of the syngeneic response revealed that RCS cells express inappropiate alloantigenic specificities on their surface. Thus, evidence is provided which demonstrates that RCS tumour cells carry antigens which cross‐react with BALB/c (H‐2d) and C57BL/6 (H‐2b) alloantigens. The presence of inappropriate antigens in thein vitrolines was detected by cell‐mediated cytotoxicity, cytotoxic antibody and immunofluorescence whereas antigens onin vivolines and spontaneous RCS were detected primarily by immunofluorescence. Conversely, H‐25expression on thein vitrolines was detected only by immunofluorescence whereas the H‐25of thein vivolines were detected by cytotoxic antibody. By these assays inappropriate antigens were not detected on normal SJL/J lymphocytes. Immunochemical analysis of the inappropriate antigens by two‐dimensional gel electrophoresis was performed with SJL/J anti‐BALB/c and monospecific anti‐H‐2Ddsera. These sera precipitated 45,000 MW molecules from BALB/c lymph node cells and from SJL/Jin vivoandin vitroRCS tumour which appeared very similar. The sera also precipitated molecules from SJL/J lymph node cells which resemble the BALB/c and RCS molecules. Thus, RCS tumours express inappropriate alloantigens which may be cryptic on normal cells. The biological significance of these tumour‐associated antige