This article reports the results of an investigation, based on a 170,000-word corpus of test performance, of the validity of College English Test-Spoken English Test (CET-SET) group discussion by examining the degree of interaction among candidates in the group discussion task with respect to a set of interactional language functions (ILFs) to be assessed. The results show a low degree of interaction among candidates in the CET-SET group discussion. Consequently, the inadequate elicitation of ILFs from candidates may well pose a problem for measuring their speaking ability in this regard.