Antibody-forming cells with specificities against syngeneic and allogeneic thymocytes are detected in the spleens of normal mice after activation in vitro or in vivo with lipopolysaccharide (LPS). The activity of such cells was measured in a complement-dependent plaque assay employing trypan blue dye to assess zones of lysis. Plaques were rarely seen in the absence of LPS treatment. Anti-immunoglobulin added to the plaque assay abrogated the appearance of plaques, but the addition of LPS had no effect. Furthermore, plaque formation was 2-mercaptoethanol sensitive indicating that the antibody responsible was of the IgM class. Plaque forming cells (PFC) were also detected against syngeneic and allogeneic lymph node cells and to a much lesser extent against splenocytes. The numbers of PFC found against syngeneic, allogeneic, or a mixture of thymocytes was similar and ranged from 1000 to 3000 PFC/108 viable spleen cells tested. All murine strains tested, including congenitally athymic nude mice, exhibited anti-thymocyte PFC after LPS activation. C3H/HeJ mice, genetically unresponsive to LPS, did not respond mitogenically to LPS and no anti-thymocyte plaques were observed. These findings suggest that clones of autoreactive B cells are present in normal mice and can be activated by LPS.