The serological characterization of virus isolates from verrucae vulgares and plantar warts revealed that HPV 1 and HPV 4 are present in about 50% of these warts with HPV 1 being more prevalent, especially in plantar warts. Parallel to the high incidence of HPV 1 infections, about 50% of non-selected young adults contained antibodies against HPV 1. Only HPV 4 particles, however, reacted with serum from a patient with epidermodysplasia verruciformis when tested by immuno-electron microscopy. An examination of HPV 1 proteins indicated that the major structural proteins VP2 and VP3 are trypsin sensitive. Tryptic degradation leads to distinct polypeptides with molecular weights between 37,000 and 23,000 which may be correlated to minor protein components of HPV 1 preparations. HPV 1 histone-like proteins, which co-migrate with purified cellular histones in SDS gel electrophoresis were analyzed in an acetic acid urea system. It was shown that H3- and H4-like proteins differ from cellular histones. The reason for this difference and its meaning are discussed.