The polymorphism of hyaluronidase (EC 3.2.1.35) (Hyase) was studied on a hyaluronan-polyacrylamide gel. Liver, placenta, ovary and breast tissue were found to have 7 active isoforms while leukocytes and platelets 5 and fibroblasts displayed no hyaluronidase activity. In serum, synovial fluid and urine soluble the most acidic forms are present. Desialylation showed that most of the hyaluronidase isoforms differ in the content of sialic acid. In patients with rheumatoid arthritis, hyaluronidase activity in the synovial fluid varied from not detectable to very high. A partial deficiency was demonstrated in sera from some patients with dysostosis multiplex without mucopolysacchariduria. In I-cell disease, hyaluronidase activity in serum was as that in controls.