Teamwork is an effective way of cooperative problem solving in dynamic and unpredictable application contexts. The concept of joint intention is the key of teamwork. How various speech actions can be used by software Agents to form, maintain and dissolve joint intention is a significant problem needed to be investigated. This paper aims to design a teamwork protocol based on FIPA (foundation for intelligent physical Agent) ACL (Agent communication language) which is a promising Agent communication language. To this aim, the sufficiency of the FIPA ACL in supporting Agents to form the required joint intention in teamwork is analyzed first. Specifically, the notions of the joint-request and delegation-request are distinguished and the insufficiency of the delegation-request in supporting teamwork is pointed out. Thus a new joint-request action is defined to extend the FIPA ACL. Some properties of the joint-request are also discussed. Based on the defined action, a teamwork protocol with a formal semantic description is proposed and its application is demonstrated finally. The teamwork protocol describes a new interaction pattern, which differs from those of the existing elementary request protocol, contract-net protocol and auction protocols within the FIPA Interaction Protocol Specifications. The proposed protocol can facilitate the design of interaction modules in multi-Agent teamwork.