1. Mice of A and C57BL/6 Ks strains, thymectomized at birth acquire wasting disease in 84.1% (A) and 77.1% (C57BL/6 Ks) of the cases. There is no sex predelection.
2. Anemia in these animals is characterized by shortened red cell survival and increased fragility to hypotonic salt solutions. Among thymectomized A mice reticulocytosis is absent and extramedullary hematopoiesis is found in the spleen in the presence of bone marrow hypoplasia for the erythroid and lymphocyte series.
3. Positive antiglobulin tests of the red cells were observed in all the thymectomized C57BL/6 Ks (7/7) and 71.2% of the A strains (62/87). Normal mice do not show positive Coombs' tests.
4. The globulin coat on the A strain consists of IgM, whereas ß1C and IgG are not detectable. By contrast, red cell coats of NZB mice developing spontaneous autoimmune hemolytic anemia show IgM and ß1C, but these erythrocytes do not react with anti-gamma chain antibodies. Another difference in the globulin coats of the two types of erythrocytes is that the IgM on NZB red cells has available light chain determinants but these are apparently hidden or absent in the case of sensitized erythrocytes. The difference in antibody coating, association with a component of complement in one but not the other, suggests a different mechanism for the immune surface phenomenon in each instance.
5. Anemia in NZB mice is associated with reticulocytosis while that in thymectomized A mice is not.
6. Thymectomy appears to initiate a chain of events leading to a series of autoimmune phenomena which may be due to alteration in host response consequent to loss of thymic tissue and thymic dependent functions or alternatively to infection to which increased susceptibility exists as a result of thymic extirpation.