The subgroup F adenoviruses were tested for their ability to induce interferon in semi-permissive human cells (Chang conjunctiva) and non-permissive cells (HEF, human embryo lung fibroblasts). These cells did not produce interferon spontaneously or in response to infection by either of these adenoviruses. It was concluded that interferon induction in response to subgroup F adenovirus infection is not a likely explanation of limited virus growth in culture. Adenovirus 40 and Ad 41, unlike Ad 2, were found to be sensitive to human lymphoblastoid interferon in Chang conjunctival cells. The addition of Ad 2 to cells before pretreatment with interferon resulted in the partial and complete abrogation of Ad 40 and Ad 41 interferon sensitivity, respectively. The suppressive effect of Ad 2 on the inhibitory action of interferon and the modulatory function of Ad 2 in mixed infection with either Ad 40 or Ad 41 suggests the inadequate functioning of a subgroup F adenovirus gene product or products involved in suppression of the interferon-induced antiviral state.