2,4-Diamino-6-hydroxypyrimidine and its derivatives obtained by the replacement of C-2 and/or C-4 amino group by various para-substituted arylamino or arylguanidino groups have been investigated for their inhibitory action on the growth of Streptococcus faecalis, Lactobacillus arabinosus, and Escherichia coli. 2,4-Bis(arylamino)-6-hydroxypyrimidines have been found to be the most active of the series. The biological activity of arylamino- or arylguanidinopyrimidines differs from that of 2,4-diaminopyrimidines in that the former does not competitively reverse the action of folic acid in the growth of S. faecalis. Their inhibitory activity seems to be partially due to their interference with the pyrimidine metabolism of the microorganisms. © 1961.