This study examines determinants of professional human resource management (HRM) practices within a sample of approximately 700 small to medium-sized firms. Predictions from the agency theory and the resource-based view of organizations lead to alternate hypotheses regarding the direct and indirect negative effects of family ownership and management on the usage of professional HRM practices. Results support predictions for both direct and indirect effects. These indirect effects occur through intermediary variables that reflect organizational complexity, such as firm size, (the presence of a) formal business plan, and HRM specialization. The findings lend partial support to both theories.