NMRI mice of different ages (24 hours, 10 to 12 days old, adults) were inoculated intraperitoneally with varying doses of mouse adenovirus. The susceptibility to disease was high in newborn mice (1 LD50 equalled 7 TCID50, as measured in mouse kidney cell cultures), medium in 10 days' old mice and almost nil in adult animals. The survival time was not greatly influenced by the virus dose. The susceptibility to infection is greatest in adults and lowest in 10 day old mice; these show a susceptibility equal to kidney cell cultures. In newborn mice, the virus spreads via the blood to all organs tested except for the intestinal tract, peak titers being reached in kidney and liver for at least 52 days, together with a sustained viruria. Adult mice could be infected by oral or intranasal installation.