Tests commonly used to assess the glomerular filtration rate (GFR) and to detect renal tubular damage are critically reviewed. Creatinine clearance which is frequently used for assessment of the GFR is prone to several errors. The plasma creatinine can be used to provide a rough guide but for reliable measurement of the GFR, 51Cr-EDTA clearance is recommended. Measurements of the urinary excretion of low molecular weight proteins, enzymes and kidney tissue proteins have been used to detect tubular damage. Of the low molecular weight proteins excreted, beta-2-microglobulin is unstable and measurement of retinol-binding protein or alpha-1-microglobulin is recommended for the detection of chronic renal tubular malfunction. Of the many enzymes that have been studied, urinary N-acetyl-beta-D-glucosaminidase or alanine aminopeptidase are recommended as being the most useful for the early detection of acute renal tubular damage. Among renal tissue proteins that have been measured in urine adenosine-deaminase-binding protein, a tubular brush border antigen appears to have considerable potential for providing early warning of renal allograft rejection.