The size of hyaluronan in solid tissue was measured using a combination of agarose gel electrophoresis and a radiometric assay. Radiolabeled hyaluronan binding proteins, used in the radiometric assay, were also used to detect hyaluronan after transfer to a nylon membrane following gel electrophoresis. Lane intensity on the autoradiograph was linearly related to the amount applied to the gel between 10 and 100ng. The intensity was independent of the hyaluronan molecular weight for standards with molecular weights equal to or greater than 790,000. The radiometric assay was used to measure hyaluronan irrespective of size, while gel electrophoresis was used to measure hyaluronan with molecular weights greater than 0.79x10(6) or 4x10(6). Deferoxamine was used to inhibit depolymerization during the digestion of tissue samples with protease. The molecular weight pattern was similar for skin, skeletal muscle, heart, lung, small intestine, and large intestine despite large differences in hyaluronan content. For all tissues, 58% of the hyaluronan had a molecular weight greater than 4million. All tissues showed an absence of hyaluronan with a molecular weight below 790,000. The procedure can be used to study changes in hyaluronan size in tissue during inflammation and other pathological states.