1. Further studies contrasting the effect of varying doses of 4-amino-N10-methyl-PGA on a resistant variant (Ak4R) of transplanted mouse leukemia Ak4 have been reported.
2. The usual therapeutic dosage of 4-amino-N10-methyl-PGA prevented the increase in total leukocytes, delayed the visceral infiltration, and increased survival time in the Ak4 strain but was without effect in strain Ak4R.
3. In the Ak4 leukemic mice with high leukocyte counts a marked drop within twenty-four hours could be demonstrated after a single dose of 3 mg./Kg. of this drug. With the Ak4R strain there was sufficient resistance so that a dose of at least 25 mg. Kg. was needed to show the marked leukopenia in twenty-four hours.
4. The Ak4R strain was as sensitive as the Ak4 to the leukotoxic effects of 2,6-diaminopurine. This drug also was effective in prolonging the survival time of mice with Ak4R leukemia.