Newcastle disease virus (NDV) envelope glycoproteins that are expressed at the surface of fixed NDV (Ploufragan strain)-infected chick fibroblasts induce interferon (IFN) in mouse spleen cells. HN protein appears to be involved, since an anti-HN monoclonal antibody (Mab 3115) reduces the IFN production to 6% at most. However, the precise site of the molecule responsible for IFN induction is probably not exactly superimposed on the Mab 3115-epitope, since the NDV (83309 strain)-HN protein, which exhibits a modified Mab 3115-epitope, is also able to induce IFN. These preliminary results require further investigation in order to characterize the IFN herein demonstrated, to establish whether this induction mechanism exists in chicken lymphoid cells and to more accurately define the part of the HN molecule involved.