An algorithm for finding the characterization of a class of objects on the basis of a randomly ordered sequence of labeled individual objects–some members of the class, some not–is described. The class is characterized as a disjunction of terms, each term being a conjunction of attributes. “All red, round objects or all square, small objects” is an example. Mechanisms based on this algorithm are described in terms of such properties as the amount of storage available for recording instances and the number of instances which had to be examined until the class was first guessed. © 1961, ACM. All rights reserved.