Eluates of SEYF-a cells (from polyoma vlrus-in- duced sarcoma grown In A.BY mice) containing cytotoxic antl- SEYF-a antibodies mediated complement-dependent cytotoxicity of normal lymph node cells. Some lymphocytotoxicity was also mediated by sera of tumor-bearing mice, but the pattern of cytotoxicity was different from that exhibited by tumor eluates. The titer of lymphocytotoxic antibodies in the serum was lower than the titer of anti-SEYF-a antibodies, whereas eluates exhibited similar titers toward both target cells. These results indicated that tumor cells selectively absorbed the lymphocytotoxic antibodies from the serum. Tumor eluates had cytotoxic activity also against thymocytes, splenocytes, and even bone marrow cells. The strain distribution of sensitive thymocytes to the antibodies present in eluates indicated that Thy 1 was not involved. Unfractionated eluates as well as IgG fractions isolated from eluates were active.-J Natl Cancer Inst 60: 1509-1513, 1978.