Human urinary albumin excretion (UAE) has been measured by radial immunodiffusion [1], nephelometry [2], and radioimmunoassay (RIA) [3]. RIA measures nanogram quantities of urinary albumin and, to our knowledge, is the most sensitive available method for quantitating UAE not detectable by Albustix. Recent studies argue that increased microalbuminuria in patients with insulin-dependent diabetes may be predictive of nephropathy risk [4, 5]. Because RIA poses potential health hazards, requires expensive equipment, and is technically complex, we evaluated a non-isotopic immunoassay to measure UAE. This fluorescent immunoassay (FIA) resembles the RIA, requiring specific antiserum and similar incubation and separation techniques [6]. The FIA is more sensitive than radial immunodiffusion methods and has been shown to accurately measure human albumin and immunoglobulin levels in saliva [7]. Herein, we report our studies on the sensitivity, accuracy, and reproducibility of the FIA for measuring UAE in normal subjects. Methods. Thirty-two healthy adults (14 females) hospitalized for evaluation as potential kidney transplant donors served as the subjects. Thus, BP, urinalysis, urine culture, serum creatinine, 24-hr urine collection for creatinine clearance, UAE, and an intravenous pyelogram (IVP) were obtained on all subjects. These subjects, ranging in age from 17 to 61 years (mean, 32.3 years), had normal BP (mean, 116/78 mm Hg), serum creatinine levels (mean, 0.92 mg/dl), and normal creatinine clearances (mean, 105 ml/min/1.73 m2). All other studies were within normal limits. Serum albumin levels were measured in 22 of these subjects and were normal (mean, 4.08 g/dl). Informed consent was obtained from all participants. A 5-ml aliquot from a measured 24-hr urine collection was preserved with 25 p1 of 2% sodium azide (NaN3) and stored at —20°C until evaluated. Reagents. Immunobead reagent for antibody coupling was obtained from Bio-Rad Laboratories, Richmond, California. Both fluorescein isothiocyanate (FITC) conjugated and unconjugated goat antisera to human albumin (GAHA) (IgG fraction) were purchased from Cappel Laboratories, Cochranville, Pennsylvania. Chicken egg albumin grade V and human albumin were obtained from Sigma Chemical Company, St. Louis, Missouri. Lyophyllized immunobead matrix and antisera to human albumin were reconstituted according to package in-