One of the more recent cognitive theories concerning learning concepts, referred to as prototype theory, suggests that categorization is enhanced by prior formation of a prototype, ie a typical instance of a category. This article describes an experiment in which the differential effects of prototype formation on the classificatory behaviour are assessed. The experiment is orientated to categorization of a disease. This is a complex phenomenon as a result of the fact that the boundaries of most diseases are fuzzy. The instructional design guide for teaching concepts, as developed by Merrill and Tennyson (1977), underlies this experiment. It provides prescriptions for the learning of fuzzy concepts. The objective to be learned is the correct identification of different types of the concept shock, ie a state of circulatory insufficiency in which cellular perfusion is inadequate for normal cellular function. Data provide evidence for the validity of the prototype paradigm and of the utility of the instructional design model of Merrill and Tennyson.