Rabbits immunized to different heterologous renal antigens developed antibodies some of which are fixed to their own glomeruli (autoantibodies). These autoantibodies, reacting with the host's kidneys, were directed to those antigenic determinants which were present in identical or cross-reactive form in both the immunizing antigen and the kidneys of the host. Glomerulonephritis developed in some of the rabbits immunized to heterologous mammalian kidneys, but in none of those immunized to nonmammalian kidneys. The development of glomerulonephritis in the immunized rabbits appears to depend upon two factors: (a) the amount of autoantikidney antibody made to the common renal antigens; and (b) the quantity of the common antigens available in the host's kidneys. An amount of common antigen sufficient to fix a nephritogenic amount of antibody is essential for the development of disease.