Graduate teaching assistant (GTA) training initiatives such as the Lead TA Program seek to enhance the instructional competence of GTAs at a disciplinary level. This paper outlines the results of a mixed-method study conducted to evaluate the perceived impact of the Lead TA Program on GTAs during a two-year pilot implementation stage at a large, research-intensive Canadian university. As a result of participating in programming offered by Lead TAs, GTAs reported overall gains in their confidence as an instructor as well as increased disciplinary instructional competence. GTAs' perceived benefits in relation to disciplinary instructional competence included: (a) increased knowledge of the TA role in the context of their department, (b) gains in pedagogical content knowledge, and (c) increased classroom management skills when facilitating disciplinary tasks or discussions. The study points to the potential for the Lead TA Program to enhance the general, domain, and topic-specific pedagogical content knowledge of GTAs. Unique challenges of implementing discipline-specific programming are addressed and recommendations are offered for establishing similar programs at other universities.