Murine C-type viruses can be activated by immunological reactions such as graft versus host reactions, mixed lymphocyte cultures, and skin allograft rejection reactions. We will attempt to evaluate what the controlling factors are governing such activation, and to see whether immunological activation of viruses also occurs in man. We will also explore the relationships between virus carrier states and the immune response in mice that are infected with C-type viruses at birth or by transplacental infection.