This paper describes NEOMYCIN, a computer program that models one physician's diagnostic reasoning within a limited area of medicine. NEOMYCIN's knowledge base and reasoning procedure constitute a model of how human knowledge is organized and how it is used in diagnosis. The hypothesis is tested that such a procedure can be used to simulate both expert problem-solving behavior and a good teacher's explanations of reasoning. The paper is divided into four major sections: (1) acquiring the model by protocol analysis, using a framework that separates an expert's causal explanations of evidence from his descriptions of knowledge relations and strategies; (2) overview of the diagnostic model; (3) representing the model, including strategy and domain knowledge; and (4) evaluating the model for sufficiency and plausibility by testing it in different settings requiring expertise. NEOMYCIN's diagnostic procedure is discussed in detail, as a memory aid, as a set of operators, as proceduralized constraints, and as a grammar. (Author/LMO)