At least two groups of human papilloma viruses can be distinguished serologically: on the one hand HPV 1-3, which are closely related but differ in the restriction enzyme pattern of their DNA, and on the other hand HPV-4. The age distributions of patients with warts induced by HPV 1-3 or by HPV-4, respectively, differ markedly. HPV 1-3 predominates between 5 and 15 years of age, whereas HPV-4 could be isolated more often between the ages of 20-25 years. The large number of HPV-1-3-induced warts in children is paralleled by a high percentage of HPV-1 antibody-positive sera in the same age group (about 50%). With increasing age the percentage of HPV-1 antibody-positive sera decline gradually. This pattern of seroreactivity was compared to that of patients with various papillomas and with several malignant tumors. There is no evidence to suggest a link between HPV 1-3 and condylomata acuminata, laryngeal papillomas or any of the malignant tumors tested.