The authors report the results of immunofluorescence (IF) studies of 17 cases of "non-idiopathic" renal biopsy-proven amyloidosis and 18 cases of various nephropathies and normal kidneys (as controls), investigated by IF by simultaneous use of antisera against routine IgG, IgM, IgA, C3, C4, Clq, beta-lipoprotein, albumin, and fibrinogen. Antisera against kappa and lambda light chains and amyloid A and amyloid P components were also used. Six of the 17 cases of amyloidosis were associated with immunocyte dyscrasia, and 11 were cases of reactive systemic amyloidosis associated with chronic infections or inflammatory and neoplastic disorders. In amyloidosis, IF deposits appeared for all antisera as homogeneous staining of mesangial nodules, and, more rarely, there was staining along the glomerular basement membranes. Overall immunoglobulins and C3 were present in 11 cases (64 per cent). Kappa and lambda light chains were demonstrated in 14 (82 per cent) and 12 (70 per cent) cases, respectively. In immunocyte dyscrasia associated with amyloidosis, immunoglobulin and light-chain deposits corresponding to a paraprotein abnormality were demonstrated in glomeruli and in tubular casts. Amyloid P component was always present in glomeruli with a bright and characteristic fluorescence, and it was frequently observed in arterioles. Amyloid A component was observed in six cases of reactive systemic amyloidosis but also in one case of immunocyte dyscrasia with amyloidosis. In view of the diversity of amyloid fibril types and their chemical nature, IF studies confirm the presence of different constituents but do not warrant any conclusion concerning the pathogenesis of this disease.