The autoimmune, short-lived, NZB/BINJ mice were followed for the changes taking place in the number of thymic and splenic nucleated cells from day 1 up to 12 mo of age. The long-lived C57BL/6J mice were monitored in parallel with the NZB mice to see whether similar changes were reflected. Both strains of mice were investigated for the sequential appearance of anti-erythrocyte autoantibody classes by the direct Coombs' agglutination test. IgGl was the first class of antibody detected on the erythrocytes and was followed by IgG3, IgG2 and IgA, simultaneously. IgM was the last antibody to react with erythrocytes. The C57BL strain had a very small number of mice with low levels of IgG1 antibody on erythrocytes throughout the study.