INTRODUCTION By the late nineteenth century, formal exchanges of scientific materials operated among federal institutions, but they did not include medical publications. North American leaders in medicine therefore called for their own centralized exchange, to improve their libraries through weeding duplicates, completing runs of journals, and obtaining key books. Rather than establish physical quarters, such as a clearinghouse, they took a novel approach by creating a society to coordinate an exchange of publications: the Association of Medical Librarians, later called the Medical Library Association (MLA). As shown in Guardians of Medical Knowledge, a study of MLA’s first fifty years, MLA was originally founded as a consortium of medical libraries to run an exchange for its institutional members and to pressure publishers to donate medical literature to them. Of the two earliest classes of membership, Library Membership took priority over Individual Membership, and MLA did not become an association in which individual memberships predominated until after World War II. In short, there were really two MLAs: one before 1946 and a very different one after 1946 .