Among the sources of teachers' self-efficacy beliefs, mastery experiences are postulated to be the most potent. Thus it seems likely that other sources of self-efficacy would play a larger role early in learning when fewer mastery experiences are available. Among the 255 novice and careers teachers who participated in this study, contextual factors such as the teaching resources and interpersonal support available were found to be much more salient in the self-efficacy beliefs of novice teachers. Among experienced teachers, for whom an abundance of mastery experiences were available, contextual factors played far less important a role in their self-efficacy beliefs.