Induction of cell mediated immunity in the respiratory tract after intratracheal immunization with live or killed influenza A virus was investigated. Respiratory ciliated epithelium appears to be one of the targets involved in the expression of local cell mediated immunity. The degree of inhibition of ciliary movement registered after contact with the antigen is compared with the degree of the development of cell mediated immunity in the lower respiratory tract as evaluated by macrophage migration inhibitory factor (MIF). It was found that 82% of the animals with MIF showed inhibition of ciliary movement on contact with the antigen. From the cytologic point of view, alterations in the ultrastructure of the cilia occur very quickly, within a few minutes. After contact with the specific antigen first the membranes of the cilia are destroyed and later on the cilia themselves. A lymphokine is proposed as a mediator of the inhibition of ciliary movement as well as a cytotoxic effector. This is because the sensitized lymphocyte culture supernatants as well as pulmonary lavages obtained 48 h after the last local immunization also inhibit the ciliary movement.