Nowadays ICT technologies are deeply shaping the ways teaching and learning take place in the Universities. The effective exploitation of a potentially huge amount of instructional multimedia material calls for powerful search capabilities. The ideas and the tools developed in the framework of the Semantic Web can be very useful to build an enriched service for information discovery in next-generation e-learning systems. In this paper we present an ontology-based Grid Service for searching multimedia contents within our Grid-based e-learning environment. To annotate the material we have used an ontology describing the nature of the multimedia resources.