A protein which specifically complexes with adenosine deaminase (complexing protein) was localized in human kidney. Thin sections of renal tissue were treated with rabbit anti-complexing protein serum followed by fluorescein-labeled goat anti-rabbit .gamma.-globulin serum. Complexing protein was detected in the proximal tubules of 4 normal kidneys. The glomeruli in sections from 1 of the 4 normal kidneys were also positive. Lung, liver, spleen, cardiac muscle, skeletal muscle and kidney medulla were negative by this technique. The glomeruli as well as the proximal renal tubules of 2 patients with combined immunodeficiency disease-adenosine deaminase deficiency and 3 of 7 patients with kidney disease contained easily detectable levels of complexing protein.