High doses (>1.56x10(7) p.f.u.) of purified preparations of human adenovirus types 3, 5 and 8 exhibited definite pyrogenic activity when injected intravenously into rabbits. Complete pyrogenic tolerance was obtained not only with homologous types but also with heterologous types of adenovirus. No pyrogenic cross-tolerance was observed between each of these three adenovirus types and paramyxovirus pyrogen or bacterial lipopolysaccharide. Adenovirus pyrogenicity was retained after UV-inactivation, whereas it was inactivated by heating at 56 degrees C for 30 min. Adenovirus pyrogenicity was not neutralized by mixing with homologous type-specific antiserum but non-pyrogenic doses (10(7) p.f.u.) of adenovirus types 3, 5 and 8 became highly pyrogenic in the presence of type-specific antibodies at the optimal virus:antibody ratio. This enhanced pyrogenicity depended upon the virus-antibody complex. From these results, it is probable that the pyrogenic activity of the virus-antibody complex, rather than the pyrogenic activity of the virions, is the main contributor to fever in adenovirus infection under actual physiological conditions.