TESSA is an interactive translation system designed to support transactions between a post office clerk and a deaf customer. The system translates the clerk's speech into British Sign Language (BSL), displayed on a screen, using a specially-developed avatar (virtual human). TESSA is a context-constrained exemplification of one of two basic approaches to machine translation, neither of which can currently fulfil all of the demands of successful automatic translation. Drawing on recent research in theoretical psycholinguistics, we show how TESSA is a convincing prototype model of one aspect of real human language processing. Ways are suggested of exploiting this parallel, potentially offering new possibilities for the future design of artificial language systems.