Abstract: The method is based on the agglutination, by protein, of calibrated latex particles coated with a specific antibody. The automated system consists of a modified sampler, a peristaltic pump, a thermostatic bath, a manifold, an optical cell counter equipped with a double-threshold system, and a recorder with a continuously adjustable scale. The reaction mixture (sample plus antibody-coated latex beads) is incubated in a heated mixing coil for 25 min, and the extent of agglutination is measured with the cell counter. No external shaking is required. A wide variety of proteins in plasma and urine, including human ferritin, β2-microglobulin, retinol-binding protein and albumin, have been determined. The detection limits range from 1 pM to 0.1 nM. Within- and between-assay coefficient of variation are <10%. In the assay of ferritin, sera are pre-treated to prevent interference from chylomicrons, complement and rheumatoid factor.