The colonization of adjacent professional fields has been considered as crucial to understanding the success and influence of large accounting firms, such as the Big 4. Yet, given the complexities of managing different professional groups, remarkably little is known about the internal dynamics behind large multidisciplinary accounting firms’ external responses to institutional pressures. In this article, we show how exogenous coercive pressure, such as regulation (in this case Dutch accountancy regulations), not only affect the day-to-day work of accountants, but also that of non-accountants such as tax advisors. From the perception of the tax advisors who confront regulations which are not “theirs”, we show how their internal responses evolve and tread a fine line between contestation and collaboration with their colleague accountants/auditors. Using a boundary work perspective, we examine this shift in responses and explain how tensions between professional groups may be reduced. Overall, our study not only furthers our insights into the internal dynamics behind PSFs’ (Professional Service Firms’) external responses, but also sheds light on why professional groups stay on board despite unfavorable internal conditions.