"Currently, forty WTO Members are covered by the WTO Agreement on Government Procurement. These comprise: Canada; the European Communities, including its 27 member States; Iceland; Israel; Japan; (South) Korea; Norway; Singapore; Switzerland and the United States" and several other smaller countries, e.g. Lichtenstein.
Parties to the agreement do not discriminate against each other's bids on national (federal) procurement projects. Bids from countries that do not belong to the agreement have no such protection and can be ignored or tossed out or ignored, if desired.
http://www.wto.org/english/tratop_E/gproc_e/overview_e.htm#gpa