Many preschoolers from low socioeconomic-status (SES) backgrounds demonstrate lags in their language development, and preschool participation is viewed as an important means for mitigating these lags. In this study, we investigated how teacher-child relationship quality and children's behavior regulation within preschool classrooms were associated with grammar gain for low-SES preschoolers. Direct child measures and indirect teacher reports were used to assess child language gain, teacher-child relationship quality, and behavior regulation for 173 preschool children enrolled in targeted-enrollment preschool classrooms. Hierarchical linear modeling showed a positive link between close teacher-child relationships and preschoolers' grammar gain during the school year. Interestingly, a significant interaction between conflicted teacher-child relationships and children's behavior regulation indicated that multiple factors are associated with children's language development in the area of grammar. Nurturing classroom environments and strong behavior regulation abilities may act as protective mechanisms for preschoolers from disadvantaged backgrounds. (Contains 4 tables and 1 figure.)