Intracerebral (i.c.) infection of adult mice with lymphocytic choriomeningitis (LCM) virus can result in acute lethal central nervous system (CNS) disease which is the result of the host's thymus-derived lymphocyte (T cell) response against the virus. Whether the specific effector function of the T cell is that of a cytotoxic cell (Tc) or a delayed-type hypersensitivity cell (Td) is still under debate. We assumed that if Td cells were important in pathogenesis then accessory cells in the brain (specifically, glass-adherent macrophages) might vary with the outcome of i.c. infection. We found that accumulation of macrophages in the brain (washed from meninges and skull cap) appeared to be independent of the severity of the infection (controlled by the mouse strain as well as the strain and dose of virus used). However, differentiation of macrophages was clearly linked to whether or not the infection caused rapid death. In mice that were destined to survive, macrophages became large, extensively vacuolated, and phagocytically active. In lethally-infected mice macrophages were small and had poor phagocytic abilities. At present this dichotomy could be viewed as either a cause or a consequence of disease outcome. However, the data are not inconsistent with the hypothesis that Td lymphocytes may be of primary importance in pathogenesis.