The invention concerns allophanic acid derivatives of formula IR.sup.1 --N(R.sup.2)CO--N(R.sup.3)CO--X.sup.1 --Q--X.sup.2 --GIand pharmaceutically acceptable metabolically labile esters or amides thereof, and pharmaceutically acceptable salts thereof, in which R.sup.1, R.sup.2, R.sup.3, X.sup.1, Q, X.sup.2 and G have the meanings given in the specification. The invention also concerns processes for the preparation of the allophanic acid derivatives of formula I, pharmaceutical compositions containing them and their use as inhibitors of the binding of fibrinogen to glycoprotein IIb/IIIa.