The pathogenesis of autioimmunity in New Zealand mice is still uncertain; however, advances are being made. Genetic, viral and immunologic factors appear to be involved. The immunologic factors are complex. A very early defect is a loss of thymic regulatory or suppressor cells. These cells may keep the immune system in check and when deficient allow autoimmune responses to occur. We are trying to separate and characterize these cells with an aim toward understanding the precise cellular defect. Antibodies to T cells are produced by New Zealand mice. The role of these antibodies in the loss of suppressor cells is being evaluated. Preliminary experiments indicate that these anti-T cell antibodies preferentially eliminate suppressor cells.