Abstract: The apparent inhibitory constants (Ki app) obtained from the action on dihydrofolate reductases from Lactobacillus casei bacteria and chicken liver of fourty-seven 5-subtituted-benzyl-2,4-diaminopyrimidines (among them 4 compounds are newly synthesized)have been used for investigation of the selectivity of this kind of compound. Similar QSAR equations as the results obtained in author's previous reports are correlated using van der Waals volume of substituents (V_w) instead of MR. The selectivity of this kind of compound is disscused on the basis of QSAR correlations.