Abstract: A process for the dyeing or printing of leather with anionic metal complex dyes wherein the metal complex dyes are metal complexes of metallizable monoazo dyes or metallizable monoazomethine dyes or both metallizable monoazo and monoazo methine dyes and before the application of the dye or after the application of the dye or both before and after the application of the dye the leather is treated with a product (P) or a mixture of products (P) or an acid addition product thereof, product (P) being a polymer obtainable by reaction of(.alpha.) a mono-functional or poly-functional amine containing one or more amino groups of the group consisting of primary, secondary and tertiary amino groupswith(.beta.) cyanamide, dicyanodiamide, guanidine or biguanide or a mixture in which up to 50 mole percent of the cyanamide, dicyanodiamide, guanidine or biguanide are replaced by a di-carboxylic acid or a mono- or diester thereofwith cleavage of ammonia or a further reaction product thereof with reactant(.gamma.