College students who interact with professors and peers about academic matters have better college outcomes. Although institutional factors influence engagement, prior scholarship has not systematically examined whether class sizes affect students' academic interactions, nor whether race or first-generation status moderate such effects. We conceptualized academic interactions as forms of social capital that are sensitive to institutional characteristics. We analyzed survey data from a random sample of 346 students enrolled at a public research university linked with institutional data on student class size. We employed logistic regression on six dependent variables capturing academic interactions with professors and peers and controlled for precollege characteristics. Compared to students enrolled in smaller classes, students enrolled in larger classes had significantly fewer interactions with professors about course material and with peers about course-related ideas. Social group also moderated some effects of class size. Class size negatively influenced first-generation (but not continuing generation) students' likelihood of talking to professors or TAs about ideas from class. For discussions about future careers, larger classes had profound negative effects on Black students (for interactions with professors) and Latino students (for interactions with peers), but no effect on other groups. We discuss implications for theory and practice.