An irreducibly inconsistent system of constraints (IIS) is a minimal set of infeasible constraints. MINOS(IIS) is a modified version of MINOS 5.3 which analyzes an infeasible LP model and reports the constraints which constitute an IIS. This localization of the problem speeds the diagnosis and subsequent repair of the model considerably. The paper concentrates on the practical implementation of the filtering algorithms used for IIS localization in MINOS(IIS). New extensions to the algorithm which permit external guidance of the IIS localization are also presented.