A comparative follow-up study of the antibody response against human wart-virus was performed, using the immunodiffusion and complement fixation methods on patients with condylomas and skin warts. By the immunodiffusion method, 13% of the patients with skin warts and 3% of the patients with condylomas showed greater than or equal to fourfold increase of antibody titre during the follow-up of 2--35 months. The findings show some typical features of the weak antibody response of a chronic virus infection and suggest a serological overlapping between condyloma viruses and certain group of skin wart-viruses. The antibody prevalence in age-matched controls is shown to be significantly higher than that in the wart or condyloma patients' initial serum samples. This is indicative of the protective function of antibodies against warts and condylomas. Also the analysis of the history of warts in patients with condylomas suggests that protection against condylomas can be acquired from previous warts, evidently by immunological mechanisms. In a control group of medical students, human wart-virus antibodies were frequently (52%) found in subjects without any history of warts. This finding supports the view that human wart-virus can frequently induce latent or subclinical infections in human beings.