This article interrogates the "soft power" of teacher teamwork by probing the ways in which authority conditions the appropriation of institutional motives through collective meaning making. The study analyzes the interaction of a teacher-leader and a science teacher team across two settings of professional development organized to promote curricular reform in their U.S. secondary school. The premise of the analysis draws on frameworks from cultural-historical theories, sociological perspectives, and social semiotics to view authority as the outcome of relations of power and control. The analysis reveals how the negotiation of legitimacy in interaction functions to open up or close down possibilities for acquiring motives appropriate to subject matter, teaching, and student learning in teachers' professional practice. The article makes a novel contribution to post-Vygotskian theoretical development in its presentation of authority as an attribute of the dialectical relationship of person and society in the production of institutionalized objects. (Contains 2 tables, 4 excerpts, and 5 footnotes.)