Following a call for actor-centric research in HRM, we look at the modes how HR managers acquire HR knowledge. Our interview study with HR managers of large established companies in Japan finds a clear preference for direct inter-corporate knowledge exchanges instead of the engagement of outside consultants. Discussing our findings, we explain the reasons for this preference. Abstracting from the case of Japan, we then propose with reference to institutional theory and isomorphism that the way HR knowledge is spread is linked to the nature of employment systems. In external labour markets, HR knowledge has been reported to be predominantly spread through business consultants, educational institutions, and career changes of HR professionals, resulting in normative isomorphism. We propose that in internal labour markets, like Japan’s, companies resort to direct inter-corporate knowledge exchange, which leads to mimetic isomorphism.