This paper examines a perceived desire amongst software agent application and platform developers to have the ability to send domain-specific objects within inter-agent messages. If this feature is to be supported without departing from the notion that agents communicate in terms of knowledge, it is important that the meaning of such objects be well defined. Using an object-oriented metamodelling approach, the relationships between ontologies and agent communication and content languages in FIPA-style agent systems are examined. It is argued that for use with distributed multi-agent systems, ontologies should describe the nature of object identity and reference for each defined concept, and a UML profile supporting these modelling capabilities is presented. Finally it is shown how, given an ontology in UML, an ontology-specific object-oriented content language can be generated, allowing object structures (viewed in the abstract as UML object diagrams) to be used within message content to represent propositions, definite descriptions or (for classes without identity) value expressions.