Publisher Summary This chapter examines the autoimmune or pseudoautoimmune situations and their possible roles in the development of neoplasia. The analysis of runting is an essential preliminary to a greater understanding of neoplasia. The original “runt” disease or homologous disease was developed as a laboratory model for the study of immunological mechanisms associated with the grafting of skin and organs and the acquisition of tolerance to histoincompatible antigens. The situation leading to runt disease was originally thought to be essentially a graft-versus-host reaction (GVHR). However, host responses almost certainly occur. The situations classified as potential producers of runting syndromess include (1) experimental chimcras; (2) thymectomy and wasting disease; (3) chemical induction; (4) bacterial infection; (5) sterile bacterial vaccines; (6) subcellular fractions; and (7) viruses. Two other associated situations include experimentally induced leukopenia and the development of autoimmune reactions within the New Zealand black (NZB) strain of mice. The experimental chimeras are further subdivided into parabiotic intoxication, radiation chimeras, and the parental F1 hybrid situation. The induction of the true autoimmune reactions is yet to be verified. It may be that the extensive damage to the host's immunity system both encourages and allows an aberrant clone of cells to develop and mount an immune response directed against the host. Of the seven situations presented in this chapter only two (bacterial infection and sterile bacterial vaccines) present no immediate evidence of immunological involvement. Further analysis of the remaining five runting syndromes indicates that the direction of the permanent immunological response is against the host animal.