For teachers and educators, developing students’ interest in the classroom has been a primary concern. Relatedly, for decades educational researchers have explored the construct of interest and the factors that influence students’ interest. However, despite the importance given to context to interest development in these theoretical descriptions, less attention has been paid to understanding teacher and classmate characteristics as contextual factors that may influence, positively and negatively, students’ interest. Therefore, with the goal of exploring the role of the teacher and classmates in students’ interest in a course throughout a semester, this study explored mediator effects of students’ perceptions of teacher enthusiasm and of peer enthusiasm on the relation between their initial interest and their situational interest at the end of the semester. Also, students’ motivation for affiliation with their teacher and with peers was added to investigate associations between these student variables and their perceptions of teacher and peer enthusiasm. Data were collected in 12 different classes with different instructors. In total, 455 students participated in the study, by filling out surveys at the beginning and end of the semester. For the main analysis, a path analysis was used in order to explore the relationships among initial interest, background knowledge, perceptions of teacher enthusiasm, perceptions of peer enthusiasm, motivation for affiliation with the teacher, motivation for affiliation with peers, and the situational interest variables of “hold interest” and “catch interest” at the end of the semester. Results indicated that the relationships between initial interest and “hold” as well as “catch” interest were mediated by perceptions of peer enthusiasm but not teacher enthusiasm. Also, perceptions of teacher enthusiasm and of peer enthusiasm had direct effects on “hold” interest as well as “catch” interest. Also, students’ affiliative motivation with the teacher and peers had direct effects on their perceptions of teacher and peer enthusiasm. Overall, this study makes a contribution to an appreciation for the importance of contextual factors as well as students’ own individual variables to understanding the mechanisms by which students’ interest in a course develops and is maintained throughout the semester. / text