Protein and polypeptide derivatives and their salts are claimed characterized in that a protein or polypeptide is conjugated via an intermediate grouping containing at least one radical of the formula --C(R).dbd.N-- (or --N.dbd.C(R)--) or --CH(R)--NH-- (or --NH--CH(R)--), wherein R is hydrogen or a hydrocarbon residue which may be substituted, with the same or a different protein or polypeptide, with a reporter group or a cytotoxic agent as well as a process for their preparation and the novel intermediates therefor.