Professional collaboration is widely seen as an important component in educator professional development, but we know little about the interactional processes that undergird teacher learning in collaborative workgroups. This paper focuses on four collaborative workgroups in a yearlong, mathematics-focused professional development series for pre-Kindergarten teachers. Using a communities of practice framework, we analyzed teacher workgroup conversations to understand how these interactions mediated teachers' learning. We identified three modes of participation that characterized workgroup conversations and influenced the availability and quality of learning opportunities afforded to participants: interpersonal management, validation, and collective reasoning. Findings provide insight into how interactional processes in teacher workgroups may advance or inhibit teachers' learning.