SUMMARY.We describe a simple, rapid and sensitive homogeneous immunoassay for urinary retinol-binding protein (RBP) using latex particle-enhanced turbidimetric immunoassay. Rabbit anti-human RBP is covalently coupled to 40 nm latex particles and the assay performed on the IL Monarch 2000 centrifugal analyser, with a 20 pL sample volume and the reaction monitored at 340 nm over an 8 min period. The assay range is 0-6 mg/L with a detection limit of 25 pg/L. The within and between assay coefficients of variation are less than 1.5% and less than 2 -5 % , respectively.Comparison with radioimmunoassay for RBP showed good agreement. Additional key phrases: immunoassay; immunoturbidimetry; particle labelRetinol binding protein (RBP) is a small (21 000D) transport protein for vitamin A, which forms a complex with pre-albumin in blood but loses its affinity for prealbumin once the vitamin has been delivered to the target cells. The free RBP molecule is then rapidly filtered at the glomerulus and catabolized in the renal tubules after reabsorption by the proximal tubular cells. Other low molecular weight proteins like a,-microglobulin (a,m) and P,-microglobulin (&m) are handled by the kidney in a similar manner. In kidney disease with prevailing tubular changes these proteins are not reabsorbed and appear in the urine2Currently the most widely used test for the early detection of tubular proteinuria is the determination of urinary f12m excretion. However, the instability of this protein in acid urine is a major d r a~b a c k .~ Bernard et al. compared the urinary excretion of RBP and P2m in patients with renal disease and showed that, in the absence of a pH effect, the sensitivity and specificity of both proteins as indices of tubular function are very similar.4 Therefore in view of its greater stability in acid urine, the determination of urinary RBP may be a more practical index of proximal tubular function.Various methods for RBP measurement have been described including radial immunodiffusion