A comparison of the effects of 4-amino-N10-methyl-pteroylglutamic acid and 2,6-diaminopurine upon sensitive and resistant sublines of a strain of mouse leukemia.

sensitive strain, after continued passage through treated mice, became so m-esistant to 4-amino-N10-methyl-PGA2 that no prolongation of survival time i-csulted from the usual therapeutic cout-se of the drug. After fifteen passages through treated animals, this strain of leukemia (Ak4R) was then passed through ten generations of untreated animals with no loss of resistance.3 Similar resistance of a leukemic tumor L 1210 to several 4-amino antagonists of PGA has recently been reported l)y Law and! Boyle.4 In an attempt to quantitate this resistance a method reported previously5 for studying the effects of varying doses of 4-amino-X’#{176}-methyl-PGA on the total and differential leukocyte count was employed on the sensitive anJ resistant strains.