An antiserum against viral particles extracted from bovine papilloma has been obtained in rabbits (New Zealand). The microscopic assay was performed by the peroxidase - anti-peroxidase method on two laryngeal papillomas, four cervical condylomas, three cutaneous papillomas, one oral condyloma, as well as on four ano-genital condylomas. A positive nuclear labelling was observed in 5 to 10% of the tumor cells localized exclusively in the superficial layers as well as in the koilocytes in these tumors. The use of this antibody in immunocytochemical studies will provide an additional tool for the investigation of mucocutaneous proliferations.