β2-microglobulin, a low-molecular-weight protein, is constantly produced in the body, passes freely the glomerular membrane and is almost fully reabsorbed and catabolized by the proximal tubular cells.The relativeβ2-microglobulinicreatinine clearance is presumed to be an indicator for proximal tubular reabsorption and is measured in 48 infants and children.A 100-fold decrease during infancy and early childhood is found, representing a maturation of handling low-molecular-weight proteins by the proximal tubular cells.Since almost noβ2-microglobulin was detectable in urine with pH<6.0 and the relativeβ2-microglobulin/creatinine clearance scattered 4 to 16-fold in consecutive 6-hours urine collections, a 24-hour urine collection with oral bicarbonate loading to obtain urine pH≥6.0 is necessary to avoid erroneous interpretation.The relative flemicroglobulin/creatinine clearance is a sensitive non-invasive method to study and evaluate renal tubular damage or dysfunction in infancy and childhood.