The human papillomavirus (HPV) causes warts, but is also associated with the development of squamous cell dysplasia and carcinoma. The virus is host and tissue specific and the numerous HPV types show predilection for different body sites. Experimental production of HPV 16 particles is at present only possible using in vivo culture of keratinocytes containing episomal viral DNA. Using immunoelectron microscopy, we have investigated the localization of HPV 16 E4 and L1 proteins in a keratinized epithelium formed by grafting HPV 16-containing cervical keratinocytes onto the athymic mouse. New viral progeny are produced in this system, as confirmed by labeling of intranuclear particles with a mouse monoclonal antibody against the HPV 16 major capsid (L1) protein. The role of the E4 protein is not yet clear, although it is believed to be important for the later stages of the virus life cycle. Here we confirm its cytoplasmic localization in the cells of the spinous and granular layers and demonstrate co-localization with keratin tonofilaments.