1. A radial immunodiffusion technique was used to determine the concentrations of five rat serum proteins, slow α1- and α2-globulin, transferrin, albumin and γ2-globulin before and after the induction of immune complex nephritis.
2. Four weeks after immunization with renal tubular antigen significant increases in transferrin, slow α2-globulin and γ2-globulin concentrations were observed but albumin showed a significant fall and slow α1-globulin did not change.
3. After the development of proteinuria 7 weeks after immunization significant falls in the concentrations of albumin transferrin and γ2-globulin were noted along with significant increases in the macroglobulins, slow α1- and α2-globulin, the changes being similar to those seen in the nephrotic syndrome in man.
4. At the end of the experiment the typical ultrastructural features of immune complex nephritis were observed.