Treatment with theophylline requires monitoring of dosage, preferably by determining the theophylline content in whole blood or serum, because theophylline pharmaco kinetics is most variable and therefore difficult to define, and because of possible interactions with other drugs. In medical practice, it is important and helpful to get the results of theophylline measurements quickly so that the dosage may be corrected directly whenever necessary. Two assays for rapid theophylline determination were compared with a standard method and with each other in respect of accuracy and practicability. Both methods a suitable for practical use.