N2'-Deacetyl-N2'-[3-[(1-ethyl-2,5-dioxo-3-pyrrolidinyl)thio]-1-oxopropyl]-maytansine-d5 is labelled N2'-Deacetyl-N2'-[3-[(1-ethyl-2,5-dioxo-3-pyrrolidinyl)thio]-1-oxopropyl]-maytansine (D199030), a derivative of Mertansine (M257800) which is a cytotoxic agent. It is used as the cytotoxic component in antibody-drug conjugates.