This study investigated the effect of the interaction between test takers' background knowledge and language proficiency on their performance on the "TOEFL iBT"® reading section. Test takers with the target content background knowledge (the focal groups) and those without (the reference groups) were identified for each of the 5 selected passages based on their self-identified academic and cultural backgrounds. The test takers were further classified into high and low proficiency groups based on their TOEFL iBT scores. Differential functioning was investigated at the item, item bundle, and passage levels. The results suggested that background knowledge interacted with language proficiency on certain items, which could be attributed to idiosyncratic passage and item characteristics (i.e., characteristics that were specific to a particular passage or item). Only 1 of the 5 passages investigated showed intermediate differential bundle functioning, favoring the focal group for both the high and low proficiency groups. There was no differential functioning at the passage level. This research sheds new light on our understanding of the effects of background knowledge and its interaction with language proficiency in the context of second language reading comprehension. It also has significant practical implications for test developers in advancing fair assessments.