N-[6-(2,5-Dihydro-2,5-dioxo-1H-pyrrol-1-yl)-1-oxohexyl]-L-valyl-N5-(aminocarbonyl)-N-[4-(hydroxymethyl)phenyl]-L-ornithinamide is used as a reagent to prepare antibody-drug conjugates, which are compounds that selectively deliver cytotoxic drugs to a tumour-associated antigen.