The injection of bacterial lipopolysaccharides (Salmonella typhimurium and Escherichia coli LPS) has been shown to induce thymocytotoxic autoantibodies in various strains of mice (C57BL/6, BALB/c, DBA/2, AKR, A/J and C3HeB/FeJ). Titers up to l:16 were observed. Such antibodies did not develop in C3H/HeJ mice which are low responders to LPS. The thymocytotoxic antibodies had the following characteristics: (a) 2-mercaptoethanol sensitivity, (b) optimal reactivity at 4 degrees C, (c) cytotoxicity for autologous and syngeneic thymocytes but not for spleen cells. The cytotoxicity decreased after absorption with thymocytes, spleen cells or brain tissue but not with kidney or liver homogenates. These LPS-induced thymocytotoxic antibodies were similar to the natural thymocytotoxic antibody occurring in NZB mice.