Endotoxin (LPS) and radio-detoxified endotoxin (RD-LPS: 150 kGy 60Co-gamma irradiated) preparations were compared in mice (C57Bl X CBA: F1/Rapo) for capacity to enhance the immune response against sheep red blood cells and to induce the production of antibody against autologous (bromelain-treated) erythrocytes. As RD-LPS retains its capacity to stimulate immune response against heterologous antigen, it may be used as an immuno-adjuvant. The LPS preparation gave rise to a significant increase of autoreactive cells. However, RD-LPS activated the autoantibody forming cells only to a very small degree.