In this article, many of the reports which describe the various assay procedures for 8 of the most commonly monitored drugs in plasma (digoxin, gentamicin, phenobarbitone, phenytoin, procainamide, quinidine, salicylates and theophylline) are reviewed, together with studies dealing with interferences of other drugs with these assays. Factors which are evaluated include whether the interference was studied when the drug was taken by a patient or a volunteer or by adding it to serumin vitro,the concentration or dose of the interfering drug (when reported), and the clinical implications of the interference. Suggestions as to how to eliminate some of these potential sources of interference are made.