Abstract
1. Of 39 young and adult C3H mice, thymectomized, and a few days later inoculated intraperitoneally with passage A leukemic filtrate, none developed leukemia. Of 41 normal controls inoculated simultaneously with the same filtrate 18 (44%), developed leukemia at 7 months average age. 2. Of 55 suckling C3H mice inoculated with passage A leukemic filtrate when less than 9 days old, then 1 month later thymectomized, only 1 thus far developed a palpable spleen, and definite leukemia. Of 78 control litter mates inoculated simultaneously with the same filtrates, but not thymectomized, 68 (87%), developed leukemia at 2.7 months average age. 3. Thymectomy, either preceding, or following inoculation of leukemic passage A filtrate, inhibits development of leukemia in otherwise susceptible C3H mice.