Newborn mice infected with ts1, a mutant of the Moloney murine leukemia virus, develop neuroimmunodegeneration with death and damage of thymocytes, astrocytes, and motor neurons by 24–38 days. T cells, B cells, and astrocytes, but not neurons, are infected by the virus. Primary splenocytes and thymocytes isolated from age-matched infected or control mice, when incubated in serum-deficient media containing phytohemagglutinin-L, either homotypically aggregate and survive or swell, expose their inner membrane phospholipids, and then shrink as they fragment their nuclei and excrete DNA-containing hypoploid minicells. In our present studies, the rates of these apoptotic changes were greatly increased in the infected cells. This thymocyte death was ameliorated in vitro by addition of Th2 cytokines, but not by Th1 cytokines, or by redox agents. In contrast, death of splenocytes, which were already mitogenically activated in vivo by the virus, was prevented by Th1 and Th2 cytokines plus redox support. In vivo, this ts1-induced neuroimmunodegenerative syndrome could be completely prevented by the immunomodulator polyinosine-cytosine and partially prevented by cytokines or redox modifiers. Viral titer primarily in the brain was also diminished by polyinosine-cytosine therapy. These observations indicate that the cell death in T cells and neurons in these ts1-infected neonatal mice can be prevented in vitro and in vivo by appropriate upregulation of the immune system.