Beta 2-Glycoprotein I (beta 2GI) was identified as a major urinary protein excreted by patients with several renal tubular diseases, including adult Fanconi syndrome, nephrocalcinosis associated with autoimmune diseases, Lowe's syndrome, and Dent's disease (a familial renal tubular disease). Sixteen patients excreted between 2 and 40 mg of beta 2GI per millimole of creatinine. In contrast, 18 healthy controls had undetectable amounts of beta 2GI in urine. Isoelectric focusing followed by immunoblotting demonstrated multiple forms of beta 2GI with pls between 6.4 and 8.2. These pls are higher than for several other "tubular proteins"; beta 2GI may therefore be less retarded than more-anionic proteins by the glomerular charge-barrier. This could explain why large quantities of beta 2GI are excreted despite its relatively high molecular mass (50 kDa). Excretion of beta 2GI was easily demonstrated by routine electrophoresis of urine proteins. beta 2GI migrates in the beta-gamma region and may be confused with Bence Jones protein. beta 2GI is stable for at least two years in urine frozen at -25 degrees C.