This study was aimed at clarifying the origin and mechanism of processing of multiple chitinases detected in the culture supernatant of Bacillus circulans WL-12 grown in the presence of chitin, Re-examination of N-terminal amino acid sequences of chitinase B1 and B2 led us to propose that these chitinases are derived from chitinase D. Lysyl endopeptidase digestion of both chitinase B1 (39 kDa) and B2 (38 kDa) generated seven common peptide fragments which correspond to different portions of the catalytic domain of chitinase D (newly designated as chitinase D1). Peptide mapping of chitinase D1, B1 and B2 using CNBr digestion indicated that chitinase B1 and B2 are truncated forms of chitinase D1 formed by cleavage of its binding domain and fibronectin type III-like domain. Therefore, it is concluded that chitinase B1 and B2 are derived from chitinase D1 and correspond to the catalytic domain of this chitinase. Limited proteolysis of chitinase D1 with different endoproteases under non-denaturing conditions generated active molecules corresponding to chitinase B1 and B2, providing evidence that chitinase B1 and B2 are produced by proteolytic cleavage at interdomain region of chitinase D1. Results of binding experiments of chitinase D1, B2 and the proteolysed form of chitinase D1 to regenerated chitin demonstrated that the N-terminal portion of chitinase D1, lacking in chitinase B1 and B2, is important for binding activity. These overall findings suggested that chitinases of this bacterium detected to date in its culture supernatant are encoded by only three genes (chiA, chiC and chiD). All of these genes have already been cloned, sequenced and analyzed.