Interaction Grammars are a grammatical formalism which uses two fundamental concepts : underspecification and polarity. Polarities allow us to develop original methods of parsing, which are implemented in the parser LEOPAR. To test LEOPAR, we are currently developing a large scale French interaction grammar with two principles: - The grammar is completely lexicalized, so that every elementary tree description of the grammar is anchored by a word of the language. But the anchoring mechanism aims at the re-usability of lexical information. For this, we have built a syntactic lexicon which is independent of the IG formalism. - Concerning the design of the grammar, a fundamental idea, which underlies the concept of meta-grammar, is to dissociate the level at which the grammar is written from the level at which the grammar is used in NLP systems.