1. A quantitative thin-layer chromatographic procedure for theophylline has been evaluated. An internal standard, 3-isobutyl-1-methylxanthine, a compound similar in properties to those of theophylline is added to the specimen prior to extraction to eliminate the need for accurate measurements of volume during extraction or analysis. 2. This method does not show interference from other xanthines and from a number of drugs commonly prescribed with thephylline. It has an acceptable correlation with a reference gas-liquid chromatographic procedure. 3. This procedure is capable of handling batches of six or more samples faster than the serial processing of either high performance liquid chromatography or gas-liquid chromatography.