Feather follicles, excised from 6-week-old, isolator-derived, barrier-sustained chickens, that had been inoculated with both Marek's disease herpesvirus and avian leukosis virus, were examined by electron microscopy. Particles resembling herpesvirus and actively proliferating type-C virus, with budding forms, were detected. It was of interest that 2 dissimilar viruses, both associated with oncogenesis, could coexist in the same tissue.