Disclosed are new acridone dye derivatives having characteristic fluorescence lifetimes. Also disclosed are methods for labelling target biological materials employing the acridone dyes and use of the labelled materials in biological assays. The acridone derivatives have the following structure: in which Z1 and Z2 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; R2, R3, R4 and R5 are selected from hydrogen, halogen, amide, hydroxyl, cyano, nitro, mono- or di-nitro-substituted benzyl, amino, mono- or di-C1-C4 alkyl-substituted amino, sulphydryl, carbonyl, carboxyl, C1-C6 alkoxy, acrylate, vinyl, styryl, aryl, heteroaryl, C1-C20 alkyl, aralkyl, sulphonate, sulphonic acid, quaternary ammonium, the group -E-F and the group —(CH2—)nY; R1 is selected from hydrogen, mono- or di-nitro-substituted benzyl, C1-C20 alkyl, aralkyl, the group -E-F and the group —(CH2—)nY; where 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 acridone dye derivatives, each dye having a different fluorescence lifetime, the set of dyes being particularly useful for multiparameter analysis.