We find that cells infected with wild-type group C human adenoviruses are not killed by exposure to tumor necrosis factor (TNF), but cells infected with adenoviruses that delete the E3 transcription unit are highly sensitive to TNF lysis. Mock-infected cells are resistant to TNF. Thus, adenovirus infection induces cellular susceptibility to lysis by TNF, and a product of E3 protects against lysis by TNF. The E3-dependent resistance to TNF was investigated using virus mutants that delete different segments of E3. Resistance was found to depend on the presence of a 14,700 MW protein, which has only recently been identified and for which there was no known function. Our results support the hypothesis that one of the functions of TNF in vivo is to combat virus infections, and that the 14,700 MW protein evolved in adenovirus to counteract the antiviral effects of TNF.