Measurement of urinary proteins in proteinuric patients is a complex problem since the concentration, types of protein excreted, and presence of waste products vary considerably. Each method of quantitatíng urinary proteins makes certain assumptions and care must be taken to eliminate artifacts in the determinations. Concentration of urinary proteins without large losses of either all or certain ones must be achieved to quantify specific proteins. Currently, gel filtration and immunochemical methods are most often used for estimation of specific proteins or patterns of all proteins excreted. When combined, these two methods yield fairly rigorous results for the characterization and quantitation of known proteins. This approach has shown that the concept of selectivity of protein excretion does not hold over the protein size spectrum. However, the procedure is time-consuming and therefore is suited only for research studies. A more valid model to sort out the pathophysiologic basis of protein excretion may be a technique for examination of individual proteins or those of specific groups to distinguish between glomerular and tubular proteinuria. A suitable technique for this approach, because of its convenience, is that of protein electrophoresis in polyacrylamide gel in the presence of sodium dodecyl sulfate.