Protocols for multiagent interaction need to be flexible because of the open and dynamic nature of multiagent systems. Such protocols cannot be modeled adequately via finite state machines (FSMs) as FSM representations lead to rigid protocols. We propose a commitment-based formalism called Nonmonotonic Commitment Machines (NCMs) for representing multiagent interaction protocols. In this approach, we give semantics to states and actions in a protocol in terms of commitments. Protocols represented as NCMs afford the agent flexibility in interactions with other agents. In particular, situations in protocols when nonmonotonic reasoning is required can be efficiently represented in NCMs.