The aim of this paper is to analyse the relationship between trade unions' behaviour and the Central Bank's policy in a monetary union. Following Canzoneri, Henderson (1991), Jensen (1993) and Agiomirgianakis (1998), we consider a world of two equal sized countries which share the same currency but we exclude the hypothesis of international migration of labour force. Both labour markets are supposed to be controlled by trade unions whose behaviour depends directly upon the Central Bank's monetary policy. We show the differences between the two regimes: a cooperative and a non-cooperative equilibrium in which each union works independently. Concerning employment, we find that the cooperative situation is unambiguously better than the non-cooperative one.