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)═N— (or —N═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.