Wart associated antigens were observed by the indirect immunoperoxidase technique in light and electron microscopy. Positive reaction products could be found in the nuclei of wart cells, in which virus particles were labeled with a specific immune rabbit serum and with human sera from patients with warts. Tissue antigens, differing from the viral labeled inclusions, were detected within the cytoplasm of some infected cells, by means of IgM and IgG antibodies from patients with warts. In these particular cells the positive reaction occurred in irregular patterns giving microgranular precipitates, which suggests that the antigen was not uniform. No staining was observed on the cell surface. This study has shown direct immunomorphological evidence in vivo of a specific immune reaction in man directed against whole virus and wart specific cellular antigens.