This article examines how individual-level antecedents such as motivation and ability to share knowledge mediate the relationship between HR practices and knowledge-sharing behaviour. The results of a survey of 329 secondary school teachers reveal the contradictory effects of different HR practices on the mediating roles of intrinsic and extrinsic motivation to share knowledge and subsequent knowledge-sharing behaviour of teachers. The study demonstrates that opportunity-enhancing HR practices act as a moderating condition that activates either intrinsic or extrinsic motivation to share knowledge and may completely offset the effect of motivation-enhancing HR practices. The study makes a distinctive contribution by demonstrating how certain combinations of HR practices aimed to enhance knowledge sharing might in fact be a costly solution for organisations, as they activate different mediating mechanisms in the HRM–knowledge-sharing behaviour link.