Abstract  Background  Patient safety and professional self-regulation systems both rely on professional colleagues to hold each other accountable for quality of care.  Objectives  To understand how staff nurses manage variations in practices within the group, and negotiate the rules-in-use for quality of care, collegiality, and accountability.  Design/methods  Ethnographic case study, participant-observation, semi-structured interviews, policy analysis.  Setting  In-patient unit in an urban US teaching hospital.  Results  Explicit acknowledgment of conflicts and practice variations was perceived as risky to group cohesion. The dependence of staff on mutual assistance, and the absence of a system of group practice, led to the practice of "mutual deference", a strategy of reciprocal tolerance and non-interference that gave wide discretion to each nurse's decisions about care.  Conclusions  Efforts to improve professional accountability will need to address material constraints and the organization of nursing work, as well as communication and leadership skills.