Unicameral bone cyst fluid possesses N-acetyl-beta-D-glucosaminidase, beta-glucuronidase, PZ-peptidase, cathepsin D, acid phosphatase, N-acetyl-beta-D galactosaminidase, and beta-galactosidase activities. The activities of lysosomal enzymes in the cyst fluid are, as a rule, higher than in the serum, whereas the total protein content is lower. The content of collagen degradation products in the cyst fluid is higher compared to the serum. In bone cavity wall tissues, the collagen content is decreased. Adenosine 3':5'-cyclic phosphate and cyclic guanosine 3,5'-monophosphate accumulate in the cyst cavity. However, in some cases, there is no correlation among the activities of lysosomal enzymes in the cyst fluid, blood serum, and cyst wall tissues. The ratios of lysosomal enzyme activities in the cyst fluid differ from those in the cyst wall tissues, cultured skin fibroblasts, and blood polymorphonuclear leucocytes. The lack of coincidence of enzymatic spectra of the cyst fluid, wall tissues, and serum is suggestive of the diversity of ways of lysosomal enzyme enter the cyst cavity, i.e., blood, cyst fluid cells, and cyst cavity walls. The cysts with different locations (i.e., active and latent cysts) have similar lysosomal lytic potentials. The presence in the cyst cavity of extracellular lysosomal enzymes and collagen degradation products testifies to the permanent corrosion of the cyst cavity walls from the inside as well as to the increase in the osmotic pressure of the cyst fluid. Lysosome destruction should be regarded as an important pathogenetic factor that requires surgical or pharmacologic correction or both in the course of bone cyst management.