Abstract β trace, a low molecular weight glycoprotein of unknown biological function has been known for some years to be present in trace amounts in many human tissues. The present work reports purification of β trace protein by a new method and quantitation of this protein by radial immunodiffusion in concentrated urine samples obtained from patients suffering from various diseases, in particular haemopoietic malignancies. Increased levels of β trace protein were found in some but not all cases of paraproteinaemia (myeloma, macroglobulinaemia), leukaemias and lymphomas. Excretion of this protein was found to be related to the filtered load (at the glomerulus). High β trace excretion was not necessarily accompanied by high total urinary protein excretion; no definitive relationship was found between β trace excreted and any parameter of renal function, notably serum or urinary creatinine, serum urea or creatinine clearance. No association could be demonstrated between high β trace excretion and paraprotein concentration in myeloma and macroglobulinaemia, or peripheral white cell count in the leukaemias.