Anionic monoazo dyes of the formula (1) in which A represents 1- or 2-naphthyl residue, which is substituted by a total of one or two sulphonic and/or carboxylic acid groups. R.sub.1 represents hydrogen or C.sub.1-C.sub.4alkyl, each D.sub.1 and D.sub.2, independently of the other, represent either an amino acid residue resulting from removal of a hydrogen atom from the amino group of the amino acid or the residue --NR.sub.2R.sub.3, in which each R.sub.2 and R.sub.3, independently of the other, represent hydrogen, C.sub.1-C.sub.4alkyl, C.sub.2-C.sub.6alkyl which is substituted by hydroxy, halogen or cyano, phenyl which is unsubstituted or monosubstituted by hydroxy, halogen, SO.sub.3H, C.sub.1-C.sub.4alkyl or C.sub.1-C.sub.4alkoxy or alternatively, R.sub.2 and R.sub.3, together with the nitrogen atom to which they are connected, complete a saturated 5- or 6-membered ring which may contain, in addition to the nitrogen atom, one nitrogen or oxygen atom and which may be further substituted and n is 0 or 1, a process for their preparation and the use of these dyes for dyeing natural or synthetic materials, in particular, paper ##STR00001##