Many advanced grid applications need access to ontologies representing knowledge about a certain application domain. To deal with the high heterogeneity of available ontologies, we propose a general service-oriented middleware for making ontologies accessible to grid applications. Our implementation is integrated in the German D-Grid infrastructure and provides a portlet-based client access to selected biomedical ontologies such as Gene Ontology, NCI Thesaurus and several Open Biomedical Ontologies (OBO).