Antisera were prepared in (C3H/An × 1)F1 mice against a transplanted leukemia (E♀SL2) that had arisen spontaneously in C57BL/6 mice. After removal of cytotoxic antibodies to normal alloantigens by absorption in vivo in C57BL/6 mice, the antisera detected an antigen, designated E, which is distinct from the four previously recognized cell-surface antigens (G, FMR, TL, and ML) associated with mouse leukemias. E antigen was detected on two other spontaneous leukemias of C57BL/6 origin and on the long-transplanted chemically induced leukemia, EL4. It was not found on any normal C57BL/6 tissues nor on radiation-induced or on virus-induced (Gross, Moloney, Rauscher) C57BL/6 leukemias. Thirty-nine solid tumors and leukemias from strains other than C57BL/6 lack E antigen. By the indirect immunoferritin technique, E antigen was seen to occupy discrete patches on the surface of E+ leukemia cells. Murine leukemia virus (type C or enveloped A) was not observed in E+ leukemias. Intracisternal A particles were present, but, as these are found also in E- leukemias and solid tumors, it is unlikely that they are related to E antigen. Thus, the origin of E antigen, whether from cellular genes or from viral genes, is unknown.

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The authors' work is supported by NCI Grant CA 08748 and a grant from the John A. Hartford Foundation, Inc.

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