The author underlines the types and diffusion of contracts between business concerns in international trade and the other reasons that are generating an increasing evolution of the rules and customs deriving from international practice that are known as the lex mercatoria, aiming at checking their impact in relation to contracts that have no elements subject to international regulation and therefore also in domestic law. The first step is (a) an examination of the possibility that the parties entering a contracts internal to the Italian legal system have to refer to the lex mercatoria to regulate certain clauses or groups of clauses in their contracts better or in full. Next (b), the author examines the possibility to refer both entire systems of rules and principles to the lex mercatoria and even (c) the entire system of the lex mercatoria. An examination of concrete cases demonstrates that the current trend in the Italian system is positive for point (a) and negative for points (b) and (c). This trend no longer looks sustainable on the basis of rules and principles (of the domestic public order) coherent with the current phase of evolution of the Italian legal order, such that it should be possible to refer to the lex mercatoria in general, with the sole limit of the principles of domestic public order. The trend common among Italy’s judges at present is to integrate a contract with the rules and principles of the lex mercatoria, or to identify an implicit intention among the contracting parties that is deduced from rules of objective interpretation. In conclusion, the essay attempts to record the significance that a more widespread application of the rules and principles deriving from international commercial practice could have in the evolution and perfection of new rules and principles and in the rationalisation of the old ones.