A field-specific essay test was developed as an attempt to improve the ESL placement procedure for international graduate students at the University of Illinois at Urbana-Champaign (UIUC). Graduate departments were classified into four areas, business, humanities/social sciences, technology and life sciences, and a set of four input prompts, and writing questions was developed. A total of 124 volunteers participated in taking both the regularly-required general-topic test and the field-specific test. A total-group FACETS analysis of the students’ performance on the two tests showed that they performed better on the field-specific test. However, subgroup analyses showed the field-specific topic effect only in the business and life sciences subgroups, while no prompt effect was found for the humanities/social sciences and technology subgroups. Considering that early in the test development procedure, these results were predicted by in a prompt evaluation session, the results suggest that more effort should be exerted to carefully select the topic and content of prompts in order to secure equivalency of the topic effect across all disciplinary groups. This paper further addresses limitations and promising research directions.