The Protein-Protein Crosslinking Kit provides all of the reagents and purification media required to perform three protein-protein conjugations in which neither protein contains available thiol residues. Briefly, the amines of one of the proteins is thiolated with SPDP and the amines of the second protein are converted to thiol-reactive maleimides with SMCC. This kit was designed to prepare and purify protein-protein conjugates, including crosslinking phycobiliproteins (R-PE) to antibodies, but it can be modified to generate peptide-protein or enzyme-nucleic acid conjugates.