This article focuses attention on influences that affect Early Career Teachers' (ECTs') professionalism during their process of learning to teach. The main purpose is to generate an approach informed by Cultural-Historical Activity Theory (CHAT) and evaluate the relevance of this approach to research influences affecting ECTs' professionalism and retention. This approach is significant because it could be employed in future research to address a knowledge gap in the existing evidence base and to further illuminate our understanding of influences affecting ECTs' professionalism and retention. The paper introduces the key principles of CHAT. Then a context for researching ECTs' professionalism is constructed; tracing the enactment of government policy strategies within Initial Teacher Training (ITT) policies and examining the contested notion of professionalism. Next, existing empirical evidence on influences affecting ECTs' professionalism is evaluated and a knowledge gap is identified. Then an alternative approach informed by CHAT is outlined with reasons why it is useful in researching influences on ECTs' professionalism. Some limitations of employing CHAT in this approach are identified and adaptations are proposed. Argument is presented about how the gap in the existing evidence base could be addressed by utilising this approach in future research.