Our experience with the determination of theophylline in serum using a monoclonal immunoassay reagent strip test is reported. The day-to-day precision of a series of drug assay controls (n = 74) was investigated with a mean theophylline value of 15,3 +/- 1,2 mg/l. Interference and/or cross reactivity of structurally related compounds and metabolites of theophylline was tested. Serum samples from 148 asthmatic patients who were treated with theophylline were analyzed by the SERALYZER ARIS*) reagent strip test and by high pressure liquid chromatography. A correlation coefficient of 0.99 was found between these techniques using a linear regression analysis.