The use of 2,4--bis(alkylamino)pyrimidines of formula (1) R.sub.1 is C.sub.1-C.sub.12alkyl or C.sub.6-C.sub.10laryl; R.sub.2 is hydrogen or C.sub.1-C.sub.12alkyl; or R.sub.1 and R.sub.2 together form a radical of formula (1a) R' and R'' are each independently of the other hydrogen, C.sub.1-C.sub.6alkyl or C.sub.1-C.sub.6alkoxy, R.sub.3 and R.sub.5 are each independently of the other hydrogen or C.sub.1-C.sub.8alkyl; R.sub.4 is C.sub.1-C.sub.20alkyl, unsubstituted phenyl, C.sub.6-C.sub.10aryl, C.sub.6-C.sub.10aryl-C.sub.1-C.sub.6alkyl, hydroxy-C.sub.1-C.sub.6alkyl, di-C.sub.1-C.sub.6alkylamino-C .sub.1-C.sub.6alkyl, mono-C.sub.1-C.sub.6alkylamino-C.sub.1-C.sub.6alkyl, --(CH.sub.2)2-(O--(CH.sub.2).sub.2).sub.1-4--OH or --(CH.sub.2).sub.2--(O--(CH.sub.2).sub.2).sub.1-4--NH.sub.2; R.sub.6 is C.sub.1-C.sub.20alkyl, C.sub.6-C.sub.10aryl, C.sub.6-C.sub.10aryl-C.sub.1-C.sub.6alkyl, hydroxy-C.sub.1-C.sub.6alkyl, di-C.sub.1C.sub.6alkylamino-C.sub.1-C.sub.6alkyl, mono- C.sub.1-C.sub.6alkylamino-C.sub.1-C .sub.6alkyl, --(CH.sub.2).sub.2--(O--(CH.sub.2).sub.2).sub.1-4--OH or --(CH.sub.2).sub.2--(O--(CH.sub.2).sub.2).sub.1-4--NH.sub.2; or R.sub.3 and R .sub.4 and/or R.sub.5 and R.sub.6, together form a pyrrolidine, piperidine, hexamethyleneimine or morpholine ring; in the antimicrobial treatment of surfaces. ##STR00001##