This paper reports the results of a four-week questionnaire survey carried out at the National Lending Library (N.L.L.), Great Britian, to discover which types of organizations were the principal users of medical literature, what types of literature were used, and which were the main sources of references to medical publications. Industrial organizations and universities accounted for the majority (62 percent) of the loans, most of which were English-language periodicals published since 1960. For the whole sample, citation lists in periodical articles were the principal sources of references, although for literature published in the last fifteen months, abstracting and indexing journals were the main sources. Of the latter, Index Medicus proved to be the most fruitful source of references. By asking whether the item requested was really useful to their work, a measure of the reliability of the different sources of references was obtained.