  An indefinite variant of the abstract interpolation problem is considered.
Associated to this problem is a model Pontryagin space isometric operator V.
All the solutions of the problem are shown to be in a one-to-one correspondence
with a subset of the set of all unitary extenions U of V. These unitary
extension U of V are realized as unitary colligations with the indefinite de
Branges-Rovnyak space as a state space.
