This article presents an analysis showing how collegial interactions can augment the mechanism of teachers' learning from professional development. The analysis relies on social network data and self-reports of writing instructional practices from teachers in 20 different schools that were part of a longitudinal study of the National Writing Project's partnership activities. The results indicate that both organized professional development and interactions with colleagues who gained instructional expertise from participating in prior professional development were associated with the extent to which teachers changed their writing processes instruction. Furthermore, the effects of professional development varied by teachers' baseline practices. The study illustrates the potential for using data on teachers' social networks to investigate indirect effects of professional development and the variation in professional development effects associated with different initial levels of expertise. (Contains 6 tables and 4 notes.)