Unirradiated Newcastle disease virus (NDV, strain AV) induced high levels of interferon (IFN) in primary chick embryo cells if the cells were 'aged' in vitro for 6 to 7 days. Dose (multiplicity)-response (IFN yield) curves, carried out in the presence of anti-NDV serum to prevent cycling infection, revealed that stocks of NDV-AV contain about sevenfold more IFN-inducing particles (IFP) than infectious particles (PFP). These non-infectious IFP were responsible for nearly all IFN induction in 'aged' cells, since PFP were determined to be incapable of inducing IFN. In contrast, with mouse L(Y) cells as hosts, about one-third the number of particles as there are PFP appeared to score as IFP. Heat and u.v. radiation (254 nm) inactivated NDV IFP and PFP activity at the same rate whether tested in chick or mouse cells, implying that virion-associated transcription is required to induce IFN. A model is proposed to account for the generation of IFN-inducing particles from infectious NDV following u.v. irradiation, and their subsequent inactivation at high doses of radiation. The model defines a series of u.v. targets in the NDV genome that regulate the expression of IFN-inducing particle activity in 'unaged' chick embryo cells.