The GRID and MAS (Multi-Agent Systems) communities believe in the potential of GRID and MAS to enhance each other as these models have developed significant complementarities. Thus, both communities agree on the 'what' to do: promote an integration of GRID and MAS models. However, while the 'why' to do it has been stated and assessed, the 'how' to do it remains a research problem. This paper addresses this problem by means of a service-oriented approach. Services are exchanged (i.e., provided and used) by agents through GRID mechanisms and infrastructure. The paper first consists of a set of states of the art about integration approaches in GRID, MAS and Service-Oriented Computing (SOC). It secondly proposes a model for GRID-MAS integrated systems. Concepts, relations between them and rules are semantically described by a set-theory formalization and a common graphical description language, called Agent-Grid Integration Language (AGIL). This language may be used to describe future GRID-MAS integrated systems. AGIL's concepts are directly influenced by OGSA (Open Grid Service Architecture) and the STROBE agent communication and representation model.