Novel compositions (A) comprise a cosmetic medium containing soluble fluorescent dye(s) (I) and associative polymer(s) (II) selected from polyurethane, cellulose, polyvinyl lactam and unsaturated acid polymer derivatives； provided that (I) is other than a 2-(2-(4-dimethylaminophenyl)-ethenyl)-1-(methyl or ethyl)-pyridinium halide (I'). Independent claims are included for: (a) a multi-compartment kit for lightening human keratin fibers, comprising at least one compartment containing (A) and at least one other compartment containing a composition containing oxidizing agent(s)； and (b) the use of a composition (A') comprising a cosmetic medium containing (I) (without exclusion of (I')) and (II) for dyeing and lightening keratin materials.
