Feline leukemia is a disease induced by an oncornavirus infection that inevitably causes clinically affected cats to die. It has been estimated that 40% of cats are dead within 4 weeks and 70% within 8 weeks of the onset of clinical symptoms. Acemannan is a complex carbohydrate with both immunostimulatory and direct antiviral properties. Administration of acemannan for 6 weeks intraperitoneally to clinically symptomatic cats significantly improved both the quality of life and the survival rate. Twelve weeks after initiation of treatment, 71% of treated cats were alive and in good health.