A sensitive and reproducible 96-well assay of uronic acid permitting a rapid processing of a number of samples with a very low consumption of reagents is described-for the determination of complex uronic acid-bearing polyanions such as hyaluronic acid, chondroitin sulfate, dermatan sulfate and heparin. The sensitivity of the reaction was approx. 1 mug for glucuronic acid and 2 mug for complex polysaccharides, with a linear function of glucuronic acid concentration between 1 and 100 mug. The relative coefficient of variations ranged from 1.5 to 8.7% for the assay performed in the 96-well plate. These values were found to be lower than those obtained by the conventional procedure.