The resistance to herpes virus encephalitis which develops with age was studied in mice using fluorescent antibody staining. Adult mice remained susceptible to intracerebral inoculation, and the infection of the central nervous system was identical with that found in immature mice. A "barrier" to the spread of virus inoculated extraneurally developed with maturation, and the limitation of spread appeared to coincide with the infection of peritoneal and tissue macrophages.

In vitro, suckling and adult mouse macrophages were infected with equal ease. However, suckling mouse macrophages infected other cells in contact with them, while infected adult mouse macrophages did not. Studies failed to reveal the nature of this change in macrophages which developed with age.

The role of macrophages in the pathogenesis of herpes virus encephalitis is discussed. The hypothesis is made that an alteration in the macrophages of the maturing mouse plays an important role in its development of resistance to herpes virus encephalitis.

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