We recently cloned the major hyaluronidase of human plasma, which we termed HYAL1. All hyaluronidase activity could be purified from human urine on an anti-HYAL1 monoclonal antibody column. However, urine contains two hyaluronidases of 57 kDa and 45 kDa, whereas plasma only contains the 57 kDa activity. Microsequencing confirmed that both urinary isozymes have N-termini identical to plasma hyaluronidase, but a second N-terminus was found in the smaller isozyme, apparently derived from the terminal 25 amino acids of HYAL1, at the C-terminus. The two polypeptides of the 45 kDa isozyme resulting from endoproteolytic cleavage of the 57 kDa isoform are presumably linked by disulfide bonds. Sperm contains two isozymes of the testicular hyaluronidase, PH-20, and the lower molecular weight isozyme is believed to be an endoproteolytically processed form of the larger protein. Analogously to PH-20, the smaller isozyme of HYAL1 is likely to be a proteolytically processed product of the larger isozyme.