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Qualitative analysis of urinary proteins is contrasted with histological findings of 45 renal biopsies performed in patients with chronic glomerulonephritis. Compared to electrophoresis on cellulose acetate and immunoelectrophoresis, a method using polyacrylamide gel after sodium dodecylsulfate treatment makes for more refined and objective differentiation of protein abnormalities. On the whole, proteinuria of the selective glomerular or physiological type predominates in the event of minimal change or membranous lesions. The non-selective type is found more frequently with diffuse proliferative or membranoproliferative glomerulonephritis (p less than 0.025). There are, however, too many exceptions to this rule to allow certainty, and a precise diagnosis of the particular type of glomerulonephritis is thus only possible histologically. Each type of histological involvement may cause almost any of the qualitative abnormalities of proteinuria. On the other hand, qualitative analysis of urinary proteins is useful for the detection of glomerulonephritis. A glomerular type of proteinuria may sometimes reveal involvement of kidneys at a time when, quantitatively, there is no proteinuria. In cases of orthostatic proteinuria a persistent glomerular type of tracing in recumbency suggests an organic kidney ailment. All patients in this series had a glomerular type of proteinuria when excretion was pathological, thus allowing a distinction from pure tubular involvement. 10 patients of the group, however, although they clearly had glomerular lesions (3 were diffuse proliferative glomerulonephritis) showed perfectly normal proteinuria both quantitatively and qualitatively. This was the case in systemic lupus erythematosus where kidney biopsy was performed without clinical suspicion of renal involvement. In summary, qualitative abnormalities of proteinuria call attention to underlying glomerulonephritis, although no distinction can be made between the various forms and there may be no detectable abnormality even in the event of major kidney involvement.