Ukrain has previously been demonstrated to exert a malignotoxic effect in vivo. This antitumor drug has been effective in the treatment of some malignancies in experimental animals as a result of immunostimulation (macrophage stimulation). In the present study, serum chitotriosidase activity was measured as a biochemical marker of macrophage stimulation in several murine and rat models of macrophage stimulation. It was shown that zymosan, carboxymethylated glucan and Triton WR 1339 administration to CBA mice or Wistar rats was followed by a considerable increase in serum chitotriosidase activity. Murine LS lymphosarcoma development decreased serum chitotriosidase activity. Antitumor treatment by Ukrain or cyclophosphamide did not restore this index to the normal value.