Theiler murine encephalomyelitis virus (TMEV), DA strain, induces in susceptible strain of mice a biphasic disease consisting of early acute disease followed by late chronic demyelinating disease. Both phases of the disease are associated with inflammatory infiltrates of the central nervous system (CNS). Late chronic demyelinating disease induced by TMEV serves as an excellent model to study human demyelinating disease, multiple sclerosis. During early acute disease, the virus is partially cleared from the CNS by CD3(+) T cells. These T cells express Fas, FasL, negligible levels of Bcl-2 proteins and undergo activation-induced cell death as determined by TUNEL assay leading to resolution of the inflammatory response. In contrast, during late chronic demyelinating disease, and despite dense perivascular and leptomeningeal infiltrates, only very few cells undergo apoptosis. Mononuclear cells infiltrating the CNS express Bcl-2. It appears that the lack of apoptosis of T cells during late chronic demyelinating disease leads to the accumulation of these cells in the CNS. These cells may play a role in the pathogenesis of the demyelinating disease.