2,4-Diamino-5-phenylpyrimidines in the form of the free base or an acid addition salt thereof are provided having the structural formula (I): ##STR1## wherein R.sup.1 is an alkoxy, aralkoxy or a mono-substituted or disubstituted amino group, R.sup.2 is a nitro group, and R.sup.3 is an alkyl group. The compounds act as antifolate agents and are useful for therapeutic treatment, for example as antitumour agents, antipsoriatic agents, antibacterial agents, antitrypanosomal agents and antimalarial agents. Pharmaceutical preparations comprising these compounds for administration to mammals, and methods for preparing the compounds, are disclosed. Particularly useful new compounds, especially for antitumour therapy, have the structural formula IA: ##STR2## in which formula IA: n is 1-6; R.sub.1 represents hydrogen or alkyl; R.sup.4, R.sup.5 and R.sup.6, which may be identical or different, each represnt hydrogen, alkyl, alkoxy, halo, nitro, perfluoroalkyl, a group of formula --CO.sub.2 R.sup.a wherein R.sup.a represents hydrogen, alkyl or alkoxyalkyl, or a group of formula --COBR.sup.b R.sup.c wherein R.sup.b and R.sup.c which may be identical or different each represent alkyl; and R.sup.3 represents alkyl.