Our aim was to describe characteristics of GPs working in practices that achieve higher immunization coverage and more timely delivery. We conducted computer-assisted telephone interviews with randomly selected GPs from randomly selected practices. Most GPs believed immunization to be important and were confident about their knowledge of immunization although assessment revealed their knowledge to be incomplete. A minority considered six weeks is too young to immunize. Greater GP knowledge and confidence in their knowledge was associated with higher practice immunization coverage, whereas immunization delivery was more timely at practices where GPs perceived parental access difficulties a barrier.