Summary Representative myxoviruses were inactivated by cultures of normal human or mouse lymphoid cells even after stimulation with a variety of mitogens. The fractionation of cells by velocity sedimentation indicated that inactivation occurred in the presence of some lymphocyte populations but not macrophages. Pretreatment with neuraminidase prevented this effect. Cells producing antibody to sheep RBC failed either to adsorb or to inactivate virus. The direct inactivation of myxoviruses by normal lymphoid cells may be one of the mechanisms involved in host resistance against viruses of this group.