The sensitivity of interfering herpes simplex virus (HSV) particles to u.v.-irradiation was studied in a virus stock of HSV-1 strain ANG that contained an excess of interfering over infectious particles. Following u.v.-irradiation, samples of this virus stock were assayed for their plaque-forming capacity and their capacity to interfere with the replication of unirradiated standard virus. Depending on the assay conditions, interfering particles appeared to be less, equally, or more sensitive to u.v. light than infectious particles. It is concluded that interference is a gene function of interfering particles rather than being exerted directly by structural constituents of these particles.