The fastness properties of basic dyeings on cellulosic substrates are improved by treating the substrate with a fixing agent comprising a precondensate or mixture of(A) the reaction product of an amine with cyanamide, dicyandiamide, guanidine or biguanidine, or,(B) a quaternary polyalkylene polyamine with(C) formaldehyde and/or,(D) an N-methylol compound optionally together with(E) a catalyst, e.g. MgCl.sub.2.The fixing agents are normally applied as an aftertreatment, with a subsequent heat-curing step.