Formative assessment is highly beneficial for student motivation and achievement, but in undergraduate courses, especially large introductory life sciences courses, it may be difficult for instructors to gather the evidence necessary for quality formative assessment. Undergraduate teaching and learning assistants (UTLAs) can help to implement student-centered learning in undergraduate science courses, and through their roles, may provide valuable feedback to support formative assessment. Here, the authors present a model for a formative assessment system created through interactions between instructors, UTLAs, and students, and focus on a section of the model by asking: What feedback might UTLAs provide instructors and how might that feedback support formative assessment? The authors characterize and present examples of oral and written feedback provided by UTLAs to the course instructor for a large introductory Cell Biology course. An analysis of meeting transcripts and written communication over the course of a semester revealed that UTLAs offered feedback on course logistics and instructional materials, student attitudes and behaviors, and student conceptual understanding. This study provides insight relevant to those who are interested in peer-supported formative assessment in undergraduate life sciences courses and offers suggestions for how to help instructors and UTLAs collaborate to enhance formative assessment.