Rabbit anti-C3H mouse brain-associated antiserum (BA-θ) was tested for its effect on the immunologic responsiveness of preleukemic and leukemic AKR mice to sheep erythrocytes. This BA-θ antiserum was cytotoxic in vitro for both C3H and for AKR thymocytes, and was immunosuppressive in vivo. Greater immunosuppression was effected by the antiserum in preleukemic AKR mice than in leukemic animals. Control rabbit serum also was cytotoxic in vitro and immunosuppressive in vivo, but this activity was removed by absorption with homologous erythrocytes and liver tissue, and with agarose. Conversely, absorption of the rabbit anti-BA-θ serum had no effect on either the in vitro cytotoxic activity, or on the in vivo immunosuppression.
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