Immune complex deposition along the glomerular basement membrane (GBM) and in the mesangium in C57BL/6 mice bearing a transplantable syngeneic B16 melanoma is described. By immunofluorescence studies, IgG, IgM and C3 were found deposited along the GBM and the mesangium. Electron microscopy showed electron-dense deposits localized in GBM and mesangial matrix. The intensity of the immune complexes deposited in the glomeruli was correlated with progression of tumor size. In order to study the evolution of deposits, a monitoring protocol was designed. One group consisted of mice bearing B16 melanoma. They showed decreased immune complex deposition after tumor removal. Control mice inoculated with saline did not present immune complex deposition. Sham-operated mice bearing B16 melanoma showed progressive renal deposition of immune complexes. Immune complexes were made up of tumor antigen and antimelanoma antibodies and were partially characterized by elution, immunofluorescence and immuno-diffusion procedures. Antimelanoma antibodies eluted from kidneys of tumor-bearing mice were of IgG class and reacted with a soluble product extracted from B16 melanoma cells.