The effect of vibrio cholerae neuraminidase (VCN) on the growth of dimethylbenzanthracene-induced sarcoma cells in the inbred CBA mice was investigated. The use of this preparation was started after the appearance of the tumour. Injection of 50 units of VCN twice a week for three months was effective at the early stages of carcinogenesis. An increase of the life-span of mice of comparison with control animals was also observed in the animals inoculated intraperitoneally with induced syngeneic sarcoma cells pretreated with VCN and simultaneously injected (into the developing tumour) with sensitized lymphocytes received from the syngeneic tumour-bearing mice. Lymphocytes were inoculated into the growing tumour. No positive effect ensued when the lymphocytes inoculated into the tumour region were pretreated with VCN. A simultaneous inoculation of neuraminidase into the growing tumour and of syngenous induced-sarcoma cells pretreated with this enzyme intraperitoneally was the most effective. Possibilities of application of neuraminidase under clinical conditions are discussed.