A method of producing a diagnostic or therapeutic conjugate of a protein, polypeptide or peptide containing at least one disulfide bond which is necessary to maintain its biological activity, and bearing at least one thiol-containing moiety linked thereto through a hydrazone or hydrazine linkage, is effected by contacting said protein, polypeptide or peptide with a thiol-reactive diagnostic or therapeutic agent, either preformed or generated in situ, to form a stable diagnostic or therapeutic conjugate of the protein, polypeptide or peptide without substantial cleavage of the disulfide bond. Diagnostic and therapeutic conjugates produced using the foregoing method, as well as kits for carrying out the method are provided.