Early care and education (ECE) classroom quality is a critical support for children's learning and development. Evidence suggests that ECE program-level support for teachers' learning may be particularly important for ensuring high quality classrooms. Yet little is known about how such supports operate outside of resource-intensive research studies and interventions. This study uses structural equation modeling to explore several mediating pathways through which "naturally-occurring" program-level supports for professional development may be related to improved classroom quality. Results reveal a pattern of indirect associations of programs' support for mentoring with gains in both structural and interactional aspects of classroom quality the following year via teachers' participation in mentoring. Improvements in classroom quality were not similarly associated with programs' support for degree attainment nor did they operate via teachers' participation in a degree program, beliefs about teaching and learning, or job satisfaction. (author abstract)