Abstract
The development of resistance to progressive tumor growth and the effect of a thymic extract upon this developmental process were examined in CBA/Wh mice. Inoculating mice of various ages with murine sarcoma virus (Moloney) permitted the assessment of the period of time postnatally at which the animals developed a threshold level of resistance to progressive tumor growth. Resistance in CBA/Wh mice, at the virus dose used, was first detected at approximately two weeks of age and was completely developed at five weeks.Thymosin, a soluble calf thymic fraction, when administered to neonatal mice beginning on the second day of life, significantly accelerated the rate of development of resistance to progressive tumor growth. The data suggest a humoral role for the thymus in the development of tumor immunity, and that the normal development of cell-mediated immunological competence can be accelerated through the use of thymus-derived extracts.