We prepared anti-plasma hyaluronan binding protein (PHBP) mouse monoclonal antibodies and studied the fragmentation profile of PHBP with them. PHBP is present in human plasma as a single polypeptide chain (70 kDa). During the purification, PHBP partially fragmentated into the 50-kDa N-terminal fragment and the 27-kDa C-terminal fragment. After the incubation of the purified PHBP, the 70-kDa precursor form was completely cleaved to the 50- and 27-kDa fragments, followed by the 50-kDa to the 26-kDa, and the 27-kDa to the 17-kDa plus the 8-kDa fragments, respectively. Because the purified PHBP contained no other detectable proteins and PHBP has a typical serine protease domain, we concluded that the fragmentation of PHBP was caused by own serine protease activity. PHBP cleaved the C-terminal side of Arg in the peptide effectively and that of Lys weakly. The results of the pre-incubation experiments of PHBP suggested that the single-chain form of PHBP is a precursor, the two-subunit structure is an active form and the three- or four-chain structure is an inactive form of a serine protease.