Summaryβ2-microglobulin excretion in 24 hour urine collections were studied in patients with upper and lower urinary tract infections and the results compared with normal subjects. In upper U.T.I. the 24 hour excretion ofβ2m was significantly increased as the values in patients with only a cystitis were completely normal. There was no overlap between the 2 groups. Preliminary studies have been performed to compare the values ofβ2-m excretion with immunofluorescence of antibody coated bacteria for the diagnosis of pyelonephritis.