Abstract: According to the principle of isosterism the-CH₂-group of 5-(substituted benzyl)-2, 4-diaminopyrimidine was modified by—S—and—Se—and some 5-(substituted phenyl)thio-2, 4-diaminopyrimidines and 5-(substituted phenyl)seleno-2,4-diaminopyrimidines were synthesized. Their inhibitory activities on L. casei and chicken liver dihydrofolate reductase were determined. Preliminary data showed that the inhibitory activities of these compounds were less than those of the corresponding 5-(substituted benzyl)-2, 4-diaminopyrimidines. Their selectivities are also decreased.