Detecting and quantifying hyaluronan (HA) made by Class I HA synthase (HAS) and determining the level of activity of these membrane-bound enzymes is critical in studies to understand the normal biology of HA and how changes in HAS activity and HA levels or size are important in inflammatory and other diseases, tumorigenesis, and metastasis. Unlike the products made by the vast majority of glycosyltransferases, HA products are more complicated since they are made as a heterogeneous population of sizes spanning a broad mass range. Three radioactive and nonradioactive assay methods are described that can give the amount of HA made with or without information about the distribution of product sizes.