This study examined how the social interactions that mathematics teaching assistants (TAs) have within their institution influenced their professional identity development as early-career undergraduate instructors. We drew on a sociocultural perspective of professional identity development in higher education to examine TAs' interactions with students, faculty, and other TAs. We qualitatively analyzed five mathematics TAs' responses to semi-structured interviews and found that some dimensions of their identities were more frequently situated within specific relationships, while others were evident in multiple relationships. Overall, the social interactions were sites for professional identity development. Identity is a complex construct, and a better understanding of how professional identity is developed can inform higher education institutions on ways to support positive identity development of future mathematics instructors. [For the complete proceedings, see ED630210.]