A study examined the application and availability of clinical and biomedical knowledge in the clinical reasoning of physicians as well as possible mechanisms responsible for changes in the organization of clinical and biomedical knowledge in the development from novice to expert. Subjects were 28 students (10 second year, 8 fourth year, and 10 fifth year) and 10 family physicians with at least 4 years of experience. Subjects were presented with either a typical or atypical case of pancreatitis which they were to diagnose while thinking aloud and then to describe by writing out the pathophysiological processes they believed to underlie the case. Level of expertise correlated with number of case findings subjects responded to with knowledge application propositions. Fourth year students applied the most knowledge propositions and fifth year and experienced physicians the least. Results suggested that the overt role of biomedical knowledge in expert clinical reasoning is virtually absent, though this knowledge has not decreased but increased. Comparison of the overlap between applied and available knowledge suggested that, in approaching the unstructured information of the atypical case, clinical knowledge was preferred over biomedical knowledge. It is concluded that biomedical knowledge becomes increasingly integrated in the clinical knowledge base with increasing expertise. Contains 11 references. (DB)