A new low molecular weight protein was purified from the urine of uraemic patients. The protein was found to be glycoprotein with a molecular weight of 31 500, determined by SDS-polyacrylamide gel electrophoresis, and a carbohydrate content of 19%. It was electrophoretically heterogenous and migrated in the slow alpha1-region. The mean serum level in 20 apparently healthy individuals was 32+/-10 mg/1. The serum level was normal in 5 patients with severe reduction of plasma protein synthesis caused by cirrhosis of the liver but elevated in patients with a decreased glomerular filtration rate. The mean urinary excretion in 5 healthy individuals was 1.3 mg/24 h. Increased excretion was seen in 10 patients with varying degrees of uraemia.