This study explores the relationship between teaching assistants (TAs) and student learning in undergraduate science laboratory classes. TAs typically instruct laboratory courses, yet little, if any, research examines professional development (PD) for TAs or relationships between instructors and students in laboratory settings. The use of undergraduate TAs (UTAs) in the same manner as graduate TAs (GTAs) in inquiry-based lab settings has yet to be explored. This quantitative study explored how TAs' content knowledge, beliefs about teaching, and teaching confidence change as a result of PD and how TAs' prior experience, UTA/GTA status, content knowledge, beliefs, and teaching confidence relates to students' content knowledge learning in an inquiry-based general chemistry laboratory. Participants included 14 GTAs, 5 UTAs, and their 529 students at a public university. PD supported TAs to lead inquiry-based general chemistry laboratory classes, involving a week-long workshop and 14 weekly follow-up meetings. Results demonstrate that TAs' content knowledge improved following PD and teaching, t(18) = -3.62, p = 0.002, and students' content knowledge significantly improved across the semester, t(528) = -36.27, p = 0.000, d[subscript Cohen] = 1.3. Further, TAs with higher content knowledge post-PD tended to have students with higher end-of-semester content (r = 0.517, p = 0.000). No differences existed between UTAs or GTAs on any TA characteristic or student outcome measure. Using a hierarchical linear regression model, student post-semester content knowledge was predicted by student demographics; however, no TA characteristics or demographics were significant predictors of student content knowledge. Students who perceived their TA as more supportive also believed they learned more content (r = 0.280, p = 0.000). Thus, UTAs can be used in lieu of GTAs in our inquiry-based general chemistry laboratory context and could be a possible alternative for TA instructors at other universities. Continued work examining TA PD, TA characteristics, TA practice, and student learning in inquiry-based laboratory contexts is warranted.