L2C leukemic cell suspensions consistently induce a generalized and fatal stem-cell leukemia, when inoculated subcutaneously or intraperitoneally into young, adult ‘strain 2’ or F1 hybrid guinea pigs. In experiments here reported ‘strain 2’ and F1 hybrid guinea pigs were inoculated by superficial skin scarification with small doses of diluted L2C leukemic cell suspensions. When about 40 to 50 days later such animals were reinoculated subcutaneously with massive doses (0.5 ml each) of leukemic cell suspensions of 10-2 concentration, 30 of 31 guinea pigs tested were found to be immune to the challenging inoculation. The same dose induced a generalized leukemia in all 45 untreated ‘strain 2’, and in 29 of 30 F1 untreated hybrid guinea pigs.