Variation of virulence of transplantable leukemias of mice on successive transfers into genetically unrelated hosts.

Owing to differences in histocompatibility genes, most transplanted mouse tumors grow pro gressively, as a rule, only in animals of their strain of origin. In mice of genetically unrelated strains they usually grow for a variable period of time and subsequently regress. As the number of serial transfers increases, some transplantable tumors, besides killing 100 per cent in their own strain, ac quire the ability to grow progressively and kill a certain percentage of animals of other unrelated strains. In other words, they become less specific. Concurrent with this increased virulence, a de crease in histocompatibility gene requirements has been observed. Extreme cases of this kind are the so-called "nonspecific" tumors, such as the