Amiodarone-associated pneumonitis is now a well-known clinical entity, but the mechanism for the induction of the pulmonary disease is ill defined. In four patients with this disorder, evidence was obtained for elaboration of a lymphokine, leukocyte inhibitory factor (LIF), by peripheral blood lymphocytes after incubation with amiodarone in the direct leukocyte migration inhibition test. Control lymphocytes from normal subjects, as well as from patients receiving amiodarone but without pneumonitis, failed to elaborate LIF in the presence of the drug in this test. This production of LIF suggests that pneumonitis associated with amiodarone therapy is also associated with a specific cellular immune response to the drug.