The concentration of serum C3 in quails bearing tumors induced by Rous sarcoma virus (RSV) was elevated in parallel with tumor growth, whereas serum C3 levels in quails inoculated with avian leukosis virus, which lacks transforming activity, showed a pattern similar to that in mock-infected quails. C3 deposition was also observed in almost all tumor cells at the tumor developing stage. These findings obtained in vivo suggest that the cells transformed by RSV activated the C3 on their surfaces.