In vitro treatment of Lewis lung carcinoma cells with the polysaccharide levan was shown previously to reduce the oncogenicity of these cells by acting on the cell membrane. In the present study the direct effect of levan on the tumorigenicity of Lewis lung carcinoma in C57BL male mice was tested at different doses and intervals and temperatures of incubation. A dose-dependent decrease in tumorigenicity was observed: Doses of 0.5, 1, and 2 mg levan/2 X 10(5) cells/0.2 ml reduced tumor incidence by 30, 50, and 70%, respectively. Some degree of inhibition was observed even with a dose of 0.004 mg. The inhibitory effect was rapid; 5 minutes of incubation resulted in the same reduction in oncogenicity as 60 minutes. Inhibition at 0 degree C was as effective as inhibition at 37 degrees C. The lack of temperature effect indicates that enzymatic activity probably is not involved. The data presented here, together with previous data, suggest that levan induces a physical, rather than a chemical, change in the affected tumor cells. The polysaccharide appears to be loosely, noncovalently bound to the cell membrane.