Fresh normal chicken serum (NCS) which lacked virus-neutralizing antibody to fowlpox virus (FPV) was found to inhibit the appearance of the cytopathic effect of the virus, virus growth, and plaque formation in chicken embryo cells. Immunofluorescent examination revealed the deposition of the third component of complement (C3) on FPV-infected chicken embryo cells incubated with fresh NCS. The inhibitory activity of fresh NCS on viral cytopathic effect was independent of the Ca2+ ion and was abrogated by treatment of fresh NCS with inulin or zymosan. Similarly, deposition of C3 on FPV-infected cells occurred independently of the Ca2+ ion and was inhibited by treatment of fresh NCS with inulin or zymosan but was not inhibited by absorption with FPV-infected cells. These results suggest that antibody-independent activation of complement by FPV-infected cells via the alternative pathway caused the inhibition of the virus growth as well as the C3 deposition. Involvement of complement activation as nonspecific host response to virus infection was also suggested by the demonstration of the C3 deposition in the skin lesions of FPV-infected chickens.