New approaches to control healthcare expenditures and increase access to quality care are required by decision-makers in high-income countries. One strategy is to reallocate tasks from doctors to nurses. Evidence suggests that quality, effectiveness and efficiency of task shifting are context sensitive and affected by implementation. However, little is known about implementability of task shifting in specialised healthcare. We aimed to identify factors perceived to influence implementation of doctor-to-nurse task shifting in a hospital setting and improve understanding of task shifting implementability by using theory-based frameworks for analysing behaviour. Nurse-led bone marrow examination exemplified task shifting from the medical to the nursing domain.Doctors and nurses (n = 17) in a haematology department at a Norwegian university hospital were asked about factors perceived to influence implementation of nurse-led bone marrow aspirations and biopsies. Methods included in-depth semi-structured interviews (n = 11) and focus-group discussion (n = 6). Data were analysed using the Capability, Opportunity, and Motivation behaviour model and the Theoretical Domains Framework.Ten factors perceived to influence implementation were identified. Three factors were related to capability, including (1) knowledge and acceptability of task shifting rationale; (2) dynamic role boundaries; and (3) technical skills to perform biopsies and aspirations. Five factors were related to motivation, including (4) beliefs about task shifting consequences, such as efficiency, quality and patient satisfaction; (5) beliefs about capabilities, such as technical, communicative and emotional skills; (6) job satisfaction and esteem; (7) organisational culture, such as team optimism; and (8) emotions, such as fear of informal nurse hierarchy and envy. The last two factors were related to opportunity, including (9) project planning and leadership, and voluntariness; and (10) patient preferences.Task shifting from doctors to nurses in specialised healthcare requires not only development of technical skills but also complex changes in organisation, clinical routines and role identity. Educational and organisational interventions to build a team-oriented culture could potentially increase the possibility of successful task shifting and stimulate nurses to take on untraditional responsibilities. Environmental restructuring to support doctors using their time in activities only doctors can perform may be needed to realise potential efficiency gains.