Although mentoring research acknowledges that peers can assume an informal mentoring role, those peers face an interesting puzzle when engaging in such activities. Should they mentor rising stars who are on the fast track to success in the firm, or should they mentor perceived projects who remain works in progress in terms of their capabilities? Drawing on social exchange theory (Blau, 1964), we built a model that captures some of the different consequences (and antecedents) of informal mentoring of rising stars and perceived projects. We then tested that model in a field study with four sources (the peer mentor, the rising star and perceived project protégés, and the mentor's supervisor) and two time periods. Our results showed that the mentoring of perceived projects—not rising stars—predicted learning by the mentor. Working with both kinds of protégés was associated with positive sentiments toward the mentor on the part of the supervisor, however. Learning and supervisor sentiments went on to predict both the perceived effectiveness of the mentor and his or her organization-based self-esteem. In terms of antecedents, rational motives (e.g., creating a good impression, expanding one's capabilities) predicted the mentoring of both types of protégés, with physical attractiveness also predicting the mentoring of perceived projects. We discuss the implications of our results for applications of social exchange theory and for the practice of mentoring.