This study was designed to identify the relevant components of the organizational culture of medical group practices and to develop an instrument to measure those cultures. Building on the work of industrial psychologists and organizational sociologists, a 35-item instrument was developed through an iterative process with more than 100 medical groups. The final instrument was tested using responses from physicians practicing in two very different medical groups: one a prepaid group practice with salaried physicians and the other, until recently, a fee-for-service practice. Using stepwise discriminant analysis of the responses to this instrument, more than 90% of the physicians were able to be placed in the appropriate practice setting.