Whereas much is known about designing effective continuous professional development (CPD) for teachers, little is known about spillover effects of CPD by fostering collegial interactions. In this respect, the self-efficacy expectancy of multipliers to spread CPD issues within their own school is an important predictor for scaling. Self-efficacy can be fostered through various sources within CPD courses, including role-plays and practice phases to test issues. One essential endeavor lies in initiating collaboration in professional learning communities (PLCs). The study discussed here focuses on two courses explicitly supporting multipliers to actively engage in PLCs. In a pre/post-design, participants' development of self-efficacy was measured in relation to collaboration, and compared with that of a control group. As instrument we applied an adapted version of the self-efficacy scale by Schwarzer and Jerusalem ("Skalen zur Erfassung von Lehrer-und Schülermerkmalen", (1999), which yielded a high internal reliability (Cr alpha = 0.950). Our results point to different developments within the two specific CPD courses, with one course featuring a significant development of middle size effect (d = 0.31). In addition, developments were found to differ with respect to the duration of the CPD course.