Performance on integrated tasks requires candidates to engage skills and strategies beyond language proficiency alone, in ways that can be difficult to define and measure for testing purposes. While it has been widely recognized that stimulus materials impact test performance, our understanding of the way in which test takers make use of these materials in their responses, particularly in the context of listening-speaking tasks, remains predominantly intuitive. Recent studies have highlighted the problems associated with content-related aspects of task fulfilment on integrated tasks, but little attempt has been made to operationalize the way in which content from the input material is integrated into speaking performances. Using discourse data from a trial administration of a pilot for an Oxford English language test, this paper investigates how test takers integrate stimulus materials into their speaking performances on an integrated listening-then-speaking summary task, whether these behaviours are reflected in the relevant rating scale and, by implication, whether the test scores assigned according to this scale reflect real differences in the quality of oral performances. An innovative discourse analytic approach was developed to analyse content-related aspects of performance in order to determine if such aspects represent an appropriate measure of the speaking ability construct. Results showed that the measures devised, such as the number of key points included from the input text, and the accuracy with which information was reproduced or reformulated, effectively distinguished participants according to their level of speaking proficiency. The study's findings support the use of this particular task-type and the appropriateness of the associated rating scale as a measure of speaking proficiency, as well as the utility of the devised discourse-based measures for the validation of integrated tasks in other assessment contexts. (Contains 1 note, 5 figures and 4 tables.)