6MFA is a growth product of the fungus Aspergillus ochraceus (ATCC 28706) obtained by fermentation in stationary culture. It has both interferon inducing and antiviral properties, in vivo and in vitro, with a relatively high margin of safety (9, 17, 18). Ehrlich's ascites tumor bearing Swiss albino male mice were treated with 0.5 ml of acqueous preparation of 6MFA (0.75 mg total solids) i.p. in a therapeutic regimen schedule; the sham treated mice received only PBS. 6MFA treatment produced an increase in mean survival time over the untreated controls, restricted the body weight increase due to ascites and decreased the rate of mortality. As much as 100% of survival response was obtained in the group treated with 0.5 ml of 6MFA at the rate of one inoculation per week for 5 weeks. In general a dose-dependent response was seen in the antitumor effect of 6MFA against Ehrlich's ascites tumor in Swiss mice. Delay in administration of 6MFA to tumor bearing mice affected the survival rate.