Abstract: A selective bifunctional sequential linker has the formulaY.dbd.C.dbd.N--Q--A--C(O)--Zwherein Q is a homoaromatic or heteroaromatic ring system; A is a single bond or an unsubstituted or substituted divalent C.sub.1-30 bridging group; Y is O or S; and Z is Cl, Br, I, N.sub.3, N-succinimidyloxy, imidazolyl, 1-benzotriazolyloxy, OAr where Ar is an electron-deficient activating aryl group, or OC(O)R where R is --A--Q--N.dbd.C.dbd.Y or C.sub.4-20 tertiary-alkyl. A method for activating an amine function towards reaction with a second amine involves reacting the amine with the foregoing linker. The resultant isocyanate or isothiocyanate derivative can then be reacted with a second amine to form a urea or thiourea conjugate.The linker is useful for producing conjugates of ligands with amine-containing polymers and/or proteins, especially for forming antibody conjugates with chelators, drugs, enzymes, detectable labels and the like.