A basic class I chitinase is abundantly secreted into culture medium in cell suspension cultures of pumpkin (Cucurbita spp.) callus induced from the fruits. The chitinase was purified from the culture medium. A cDNA encoding the pumpkin chitinase was cloned by RT (reverse transcription)-PCR and RACE (rapid amplification of cDNA ends) methods. The chitinase gene was strongly expressed only in pumpkin callus cells. No chitinase mRNA was detected in intact pumpkin fruit tissues. However, chitinase was induced during callus formation from sliced pumpkin fruit tissues. Induction also occurred in the absence of 2,4-D, a chemical causing callus formation, suggesting that it may be independent of the presence of 2,4-D. Perhaps, induction is caused by wounding stress. Levels of chitinase mRNA markedly increased at 4 h after transfer of pumpkin callus cells into fresh culture liquid medium They were also high at later stages of cell suspension culture. In winged bean (Psophocarpus tetragonolobus} callus, an acidic class III chitinase was abundantly secreted at later stages of cell culture. A cDNA for an acidic class III chitinase was isolated from winged bean cells. The levels of class III chitinase mRNA were also high at later stages of cell culture. The chitinase may be induced by stress that accompanies cell senescence. On the other hand, a basic class I chitinase was specifically secreted by salt-adapted cells of winged bean. A cDNA for a basic class I chitinase was isolated from salt-adapted winged bean cells. Levels of class I chitinase mRNA was induced by NaCl. Furthermore, the mRNA was increased by adding other salts such as KCI and CaCl₂, mannitol and saccharose, but not by abscisic acid, indicating that the class I chitinase is induced by osmotic stress. Thus, the genes for class I and class III chitinases were shown to be differentially expressed and secreted in cultured plant cells. In transgenic tobacco BY-2 cells into which the pumpkin chitinase cDNA was introduced, the recombinant pumpkin chitinase was expressed and secreted into the culture medium, suggesting that the signal peptide of pumpkin chitinase also functions for secretion from tobacco BY-2 cells.