Arginine supplements were given to 6 week old CBA mice beginning 3 days prior to inoculation with a murine sarcoma virus, the Moloney Sarcoma Virus (MSV). Although the basal diet contained 1.8% arginine and was therefore not arginine-deficient, supplementation of the diet and the drinking water with 0.5% arginine HCl reduced tumor incidence, lengthened the latency period, decreased tumor size, and hastened tumor regression. Arginine also increased thymic weight and cellularity in normal and in MSV-inoculated mice. The antitumor action of arginine may be related to its effect on the thymus.