SUMMARYCBA mice, inoculated intravenously with large doses of adenovirus type 5, showed raised levels of serum aspartate aminotransferase (SAAT; EC 2.6.1.1) and died within a few days from histologically demonstrable hepatic necrosis. After inoculation of I LDs0, virus was rapidly taken up by the tissues where infectivity then declined greatly. Organ titres then increased about Ioo-fold by 48 h p.i. but, in the liver, which showed intranuclear inclusion bodies, and by electron microscopy, scattered intranuclear and intracytoplasmic adenovirions, the increase was IOOOO-to iooooo-fold. P antigen was detected by single radial diffusion in liver extracts, and by immunofluorescence in 80 % of liver cells at 36 h p.i. Hexon, penton base and fibre antigens appeared later and in fewer cells. The maximum amount of hexon, of demonstrable type 5 specificity, was shown by radioimmunoassay to be equivalent to up to 5 x io 11 whole adenovirions/g liver. It is concluded that human adenovirus type 5 undergoes an abortive but lytic infection in most liver cells but that replication may proceed to completion in a few.