U.S. universities have been employing international teaching assistants (ITAs) for more than thirty years, and their interest in using large-scale language proficiency test scores for the purpose of screening and certification of prospective ITAs has markedly grown. Although previous research generated evidence supporting the validity of such test uses, no studies closely examined the nature of ITAs’ classroom discourse in terms of functional language needed to accomplish teaching tasks and convey knowledge in instructional encounters with undergraduate students. This study approaches this gap in academic English assessment by investigating an ITA corpus containing laboratory, recitation, and lecture curriculum genres. With the underlying purpose of describing the target domain of ITA language use from a functional perspective, cross-genre and cross-discipline comparisons were conducted based on corpus data annotated using the Knowledge Framework (Mohan, 1986) in Systemic Functional Linguistics. The results revealed distinguishable patterns of functional language use in the three genres and three disciplines (Physics, Chemistry, English), also indicating that the linguistic choices are bound to knowledge structures rather than genre or discipline variables. The implications are relevant for the validation of EAP assessments intended for use in ITA contexts, and potentially for ITA training.