Forty-four 5-(substituted-benzyl)-2,4-diaminopyrimidines have been tested as inhibitors of chicken and bovine liver dihydrofolate reductase. The chicken enzyme is, on the average, about 10 times less easily inhibited than bovine enzyme. Substituents which show the greatest selectivity are 4-NHCOCH3, 3-OC4H9, 3-I, 3-CF3-4-OCH3, and 3,4,5-(OCH3)3. The inhibition constants have been used to formulate quantitative structure-activity relationships for comparative purposes.