We measured the serum concentration of alpha-1-acid glycoprotein (AAG) in 30 healthy subjects (controls), in 54 patients with various degrees of residual renal function (group I), and in 98 patients in the terminal phase of chronic renal failure (CRF) on both conservative and dialytic therapy (group II). A positive correlation between the logarithm of serum AAG and serum creatinine levels was found in group I. Serum AAG increased significantly when serum creatinine rose above 10 mg/dl. This fact would indicate that a retention of the substance occurs as the renal function falls. The mean serum concentration of AAG was significantly higher in group II patients, with no difference between those on conservative therapy and those on maintenance hemodialysis. However, levels above normal were present in only a minority of cases. We conclude that the serum AAG measurement maintains its diagnostic value as an acute phase reactant also in the terminal phase of CRF.