The interaction between language proficiency and discipline-related background knowledge is an important feature of LSAP (Languages for Specific Academic Purposes) tests and is viewed as a necessity in these tests unlike general purpose language tests which try to diminish the potential effect of discipline-related knowledge. Therefore, variables that could affect this interaction must be taken into consideration when studying a discipline-related knowledge effect. The purpose of this study is to examine the interaction between discipline-related knowledge and the objectivity of elicitation method across three language proficiency levels. The participants were 206 non-native students of civil engineering. An EAP test battery with discipline-related and discipline-unrelated reading passages and three test-task types (i.e. objective, semi-objective, and subjective) was constructed and administered. The results of a three-way factorial ANOVA indicated that discipline-related knowledge did not facilitate the performance of low proficiency students on the EAP test; however, there was a significant interaction between language proficiency and discipline-related knowledge for intermediate and high proficiency students. In addition, the proficiency effect differed across task type: test-takers could not benefit from their discipline-related knowledge during "objective" test tasks. However, "semi-objective" and "subjective" test-tasks proved well-suited for EAP tests due to the significant effect of discipline-related knowledge.