Disclosed are new quinacridone dye derivatives having characteristic fluorescence lifetimes. Also disclosed are methods for labelling target biological materials employing the quinacridone dyes and use of the labelled materials in biological assays. The quinacridone derivatives have the following structure: ##STR00001## in which Z.sup.1 and Z.sup.2 independently represent the atoms necessary to complete one ring, two fused ring, or three fused ring aromatic or heteroaromatic systems, each ring having five or six atoms selected from carbon atoms and optionally no more than two atoms selected from oxygen, nitrogen and sulphur; R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are independently selected from hydrogen, halogen, amide, hydroxyl, cyano, nitro, mono- or di-nitro-substituted benzyl, amino, mono- or di-C.sub.1-C.sub.4 alkyl-substituted amino, sulphydryl, carbonyl, carboxyl, C.sub.1-C.sub.6 alkoxy, acrylate, vinyl, styryl, aryl, heteroaryl, C.sub.1-C.sub.20 alkyl, aralkyl, sulphonate, sulphonic acid, quaternary ammonium, the group -E-F and the group --(CH.sub.2--).sub.nY; R.sup.1 and R.sup.2 are independently selected from hydrogen, mono- or di-nitro-substituted benzyl, C.sub.1-C.sub.20 alkyl, aralkyl, the group -E-F and the group --(CH.sub.2--).sub.nY; E is a spacer group, F is a target bonding group; Y is selected from sulphonate, sulphate, phosphonate, phosphate, quaternary ammonium and carboxyl; and n is an integer from 1 to 6.The invention also relates to a set of different fluorescent quinacridone dye derivatives, each dye having a different fluorescence lifetime, the set of dyes being particularly useful for multiparameter analysis.