This study examined motivation (prosocial goals), individual characteristics (sex, ethnicity, and grade), and friendship characteristics (affective quality, interaction frequency, and friendship stability) in relation to middle adolescents' prosocial behavior over time. Ninth- and 10th-grade students (N=208) attending a suburban, mid-Atlantic public high school and having at least 1 reciprocated friendship were followed for 1 year. Hierarchical regression analyses revealed that a friend's behavior is related to an individual's prosocial goal pursuit, which in turn, is related to an individual's prosocial behavior. Further, the affective quality of a friendship and the frequency with which friends interact moderate relations of a friend's prosocial behavior to an individual's prosocial goal pursuit.