In DA × Wistar F1 rats, growth of 104 Wistar-specific Sp 1 carcinoma cells s.c. was commonly prevented by a mild subclinical graft-versus-host reaction produced by injecting 50 × 106 Wistar spleen cells i.p. either concurrently with the tumor or 7 or 14 days previously. Spleen cells alone had no effect on established tumor, but their injection on Day 14 significantly reduced the recurrence rate after excision of tumor on Day 21. In vitro tests in tumor-bearing rats with graft-versus-host reactions showed increased spleen lymphocyte and serum cytotoxicity; these mechanisms may inhibit tumor growth in vivo. Because Wistar lymphocytes and Sp 1 cells are syngeneic, inhibition of tumor cannot be due to allograft rejection but is probably an effect of increased host immunoreactivity during the graft-versus-host reaction.

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This work was supported by grants from the Anti-Cancer Council of Victoria.

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