Infection of the newborn (1 day old) Swiss mouse with the virus of herpes simplex results in rapidly progressive disease culminating in death within 3–5 days. This lethal disease provides an unequivocal end point for titrations of virus and antibody. With a constant amount of virus the serum dilution end point in neutralization tests is readily reproducible. The newborn mouse is at least as effective as the rabbit or the chick embryo for the recovery of herpes virus from human sources, and possesses the advantage of indicating the presence of virus on initial passage. Susceptibility of the mouse to inoculation of herpes simplex virus by either the intraperitoneal or intracerebral route decreases with age. Antigenic analysis of 3 herpes simplex virus strains failed to show evidence of antigenic difference.