Educational reform initiatives are predicated on the professionalisation of teaching.  Professionalisation implies that teachers assume and practice increased control in areas of non-instructional decision-making, rather than being preoccupied with content and procedural knowledge. As professionals, teachers are called upon to grapple with larger educational purposes and directions.  In a more professional culture, teachers assume greater responsibility for generating their own expert knowledge. Large-scale assessment is often been portrayed as inimical to the interests of teachers and as an anathema by professional teacher associations. Little primary research has touched upon the impact of large-scale testing on teachers’ self-identity, their sense of professionalism, and their use of evaluative research.  This study examines why English teachers were motivated to take part professionally in a 1998 Canadian large-scale literacy assessment.  Teachers were interviewed before, during, and four to six months after taking part in the scoring sessions.  This paper examines an evolving and enhanced concept of professionalism among these teachers.  Rather than robbing these teachers of their professional autonomy and judgment, participation in this assessment program challenged them to reflect upon their own content and practical knowledge, critique their own teaching and evaluation practices, and redefine their professional roles. Emergent themes include affirmation and reaffirmation, validation of knowledge and classroom practice, clarification of large-scale assessment’s role in teacher and learning, enhanced professionalism, and philosophical shifts.  These teachers reconstructed professionalism and undertook professional development on an individual and collective basis and through a variety of experiences considered inimical to the collective professional welfare of teachers just a few years ago.