In this paper, the findings of immunofluorescence (IF) studies of 57 patients with childhood biopsy-proven renal amyloidosis are presented. All specimens were investigated by the direct IF technique and the simultaneous use of antisera to human IgG, IgM, IgA, fibrinogen and C3. Antisera to C1q, C4, HbsAg, IgE (in each of ten cases), kappa and lambda light chains of immunoglobulins (Igs) and albumin (in each of five cases) were also used. AA type amyloidosis was determined in all patients by Wright's potassium permanganate reaction. In thirty-four of these patients (60%), Familial Mediterranean Fever (FMF) was found to be the underlying disease for renal amyloidosis. In 39 cases (68.5%), renal biopsy showed positive fluorescence staining while in 18 cases (31.5%), fluorescence staining was negative. The immunofluorescence pattern of glomerular deposits was neither granular nor linear but large isolated or confluent masses which were located in the mesangium and in the capillary walls, and were similar in all cases whatever the antisera used. The areas showing immunofluorescence staining almost corresponded to the locations of amyloid deposits. Immunoreactants showed various combinations of deposition with the exception of IgE, HbsAg and albumin antisera which yielded continuously negative reactions. C3 was the immunoreactant most commonly encountered. Kappa and lambda light chains of lgs were demonstrated in one of five biopsy specimens tested. Although it was not diagnostic, this IF pattern was found to be rather characteristic. Demonstration of immunoglobulins and other components of the humoral immune system is not a rare occurrence in renal amyloidosis, and passive absorption of plasma proteins does not simply explain these immunohistologic findings.