The paper describes an auto-tutorial methodology for training veterinary medical practitioners to perform differential diagnoses. It describes in detail the three phases of differential diagnosis: sensory pick-up, a combination of cognition and memory; categorization, the process by which diagnosticians group symptoms and signs prior to diagnosis; and inferential strategy, a combination of what diagnosticians refer to as intuition and confirmation techniques. The paper criticizes traditional methods of classroom instruction in differential diagnosis which present practitioners with informational cues in single disease categories in symptom-grouped fashion rather than in multiple disease categories characteristic of real-life situations. Students instructed in traditional methods when confronted with a real diagnostic problem must first complete a transfer from the learned system of inforrmation (single disease categories) to the alternative system (multiple disease categories) before diagnosis can take place. The paper briefly reviews a pilot study, utilizing both graduate and undergraduate veterinary medical students, which yielded data tending to verify the transfer hypothesis. Through an understanding of the differential diagnostic process, however, simulation exercises can be constructed to teach the organization of information into multiple disease categories and the inferential skill required to utilize the multiple disease categories to reach tentative diagnoses. (Author/JR)