Biopsies from thirteen human renal allografts undergoing chronic rejection were studied by immunofluorescence microscopy. Deposits of complement were demonstrated in the basement membranes of glomeruli and tubules. The deposits at the tubular basement membranes formed irregular distinct lumps in the majority of cortical tubules and were a characteristic finding throughout the material. Similar deposits were found in arteries of different size and in arterioles. Linear deposits of IgG and IgM were found in the glomerules of seven grafts. Granular deposits of fibrinogen were found in four grafts in glomeruli and vessels. The findings could not be qualitatively distinguished from those of four renal biopsies of patients with chronic glomerulonephritis.