Most routine drug assay work is done by clinical chemists who, in general, are well aware of the importance of regular quality control checks. Few, however, have had extensive experience in analytical techniques for drugs, such as gas chromatography, and often underrate the difficulty in obtaining reliable results. Although antiepileptic drugs are among the easiest to measure, a North American study organized by Dr Charles Pippenger and a European study performed by the author demonstrated clearly the poor quality of results coming from many laboratories. From these studies have grown two external quality control schemes which together involve over 800 laboratories on a world-wide basis. Reports have indicated that the reliability of theophylline, tricyclic antidepressant and phenothiazine assays is also poor, and it might be expected that the problems will increase as attempts are made to measure drugs present in lower concentrations. Careful attention to quality control will be essential if physicians are to be provided with reliable results.