The aim of this paper is to present our model for a generic and flexible negotiation service and its implementation in Computer Supported Collaborative Work (CSCW) environments. Within a cooperative work environment, users naturally need negotiation support mechanisms to study possible alternatives in group decision making. The objective of our work is to build a negotiation model independent of any particular application field. Contrary to studied models, our model focuses on formalizing the negotiation from three points of view: exchanged information between the agents to negotiate (the language), the way this information is exchanged (the protocol), and the internal behavior of an agent (the tactics). In addition to a separation of the problems involved in each one of these three facets of the negotiation, this approach allows also a greater flexibility than traditional systems dedicated to one kind of problems. We chose to use a transactional approach based on speech acts to develop our axiom based negotiation model which has been implemented as a negotiation service in our CSCW environment DisCOO. After the tackled problem presentation, we will expose the state of the art, then the proposed negotiation formal model, and, finally, the implementation.