We have recently observed that chitotriosidase, a chitinolytic enzyme, is secreted by activated human macrophages and is markedly elevated in plasma of Gaucher disease patients (Hollak, C. E. M., van Weely, S., van Oers, M. H. J., and Aerts, J. M. F. G. (1994) J. Clin. Invest. 93, 1288-1292). Here, we report on the cloning of the corresponding cDNA. The nucleotide sequence of the cloned cDNA predicts a protein with amino acid sequences identical to those established for purified chitotriosidase. Secretion of active chitotriosidase was obtained after transient transfection of COS-1 cells with the cloned cDNA, confirming its identity as chitotriosidase cDNA. Chitotriosidase contains several regions with high homology to those present in chitinases from different species belonging to family 18 of glycosyl hydrolases. Northern blot analysis shows that expression of chitotriosidase mRNA occurs only at a late stage of differentiation of monocytes to activated macrophages in culture. Our results show that, in contrast to previous beliefs, human macrophages can synthesize a functional chitinase, a highly conserved enzyme with a strongly regulated expression. This enzyme may play a role in the degradation of chitin-containing pathogens and can be used as a marker for specific disease states.