The further integration of primary care within the wider health system is an imperative for reform in all countries. The aim of this paper is to determine the factors associated with general practitioner (GP) integration using the GP Integration Index which has been recently developed and is demonstrating good reliability. The analysis is based on a database derived from an Australia-wide mail questionnaire survey of 1874 GPs drawn from a 20% stratified random sample of 123 Divisions of General Practice (47.8% adjusted response rate). The GP Integration Index measures the level of GPs' integration with the health care system based upon a description of their own behaviour. It consists of nine GP integration factors, and their two associated higher-order factors--Primary Care Management (PCM) and Community Health Role (CHR)--as well as five GP integration enabling factors. A multivariate multilevel analysis was undertaken. An explanatory model for both PCM and CHR was tested based on the GP integration factors as well as general practice, GP and regional characteristics. CHR and PCM were most strongly associated with GP integration enabling factors (mainly at the individual-level) and, for CHR only, with urban-rural location (mainly at the area-level). The most important single explanatory variable for both PCM and CHR was the GP integration enabling factor, "Knowledge of local resources." The important explanatory variables were those reflecting the way GPs work, rather than their broad 'classification' within individual or GP-setting groupings. Based on these results, some revision to the proposed model was necessary. We conclude that processes of care factors (as compared to structure of care factors) are more important in relation to GP integration than previously recognised. Future policy initiatives to promote GP integration should focus on programs to improve GP's knowledge of local resources.