This investigation included the examination of tissues for evidence of infection with the human wart virus, and the development and application of serological techniques for demonstrating response to the infection. The majority of specimens were examined by negative staining methods, and the presence of virus particles and other structures in various types of wart at different stages is reported. Two cases of molluecum oontagiosum of the sole of the foot, previously considered a site rarely if ever infected, were diagnosed. A series of twelve genital wart specimens was studied by thin sectioning and negative staining techniques. Intranuclear virus particles with the morphology of wart virus were found in cells of the stratum granulosum, and the pancity of virus particles in such tissues noted for the first time. Examination of three specimens of bladder papillomata by such methods revealed no evidence of a virus infection. Serological studies included the application of an established precipitin test for antibody to wart virus, and development of complement fixation and passive haemagglutination tests for this purpose. The specific nature of the antigen involved in these reactions was demonstrated, and the immunoglobulin class of the antibodies concerned determined. These tests were applied to sera from 265 patients with different types of wart, and to sera from 110 people not currently infected with warts. The development of a full immune response with rising titres of antibody and appearance of immunoglobulin G was demonstrated in rabbits immunised with the virus, and in some patients, particularly those with the simple type of plantar wart. The relationship of the appearance of antiviral antibodies to regression of warts, spontaneous or following therapy, was observed and considered to be a result of increased exposure to the virus antigen at that time.