1. Theophylline metabolism was studied using seven human cytochrome P-450 isoforms (CYPs), namely CYP1A1, 1A2, 2A6, 2B6, 2D6, 2E1 and 3A4, and microsomal epoxide hydroxylase (EH), expressed in human B-lymphoblastoid cell lines. 2. At a high theophylline concentration of 10 mM four CYPs (1A1, 1A2, 2D6, 2E1) catalyzed the metabolism of theophylline. 3. Theophylline had the highest affinity (apparent Km range 0.2-1.0 mM) for the CYP1A subfamily and the kinetics of metabolic formation mediated by CYP1A2 indicated substrate-inhibition (Ki range 9-16 mM). 4. CYP1A2 catalyzed the demethylation of theophylline as well as its hydroxylation, and was associated with the highest intrinsic clearance (1995 l h-1 per mol CYP) to 1,3-dimethyluric acid (DMU). Therefore, this isoform can be considered to be the most important enzyme involved in theophylline metabolism in vitro. 5. CYP2E1 was responsible for a relatively high intrinsic clearance by 8-hydroxylation (289 l h-1 per mol CYP). The apparent Km value of this reaction was about 15 mM, suggesting that CYP2E1 may be the low-affinity high-capacity isoform involved in theophylline metabolism. 6. The affinity of theophylline for CYP1A1 was comparable with that of its homologue 1A2. When induced, the participation of CYP1A1 in theophylline metabolism may be important. 7. CYP2D6 played only a minor role and CYP3A4 was not active in the in vitro metabolism of theophylline. 8. Our findings confirm the major role of CYP1A2 in theophylline metabolism and explain why in vivo the elimination kinetics of theophylline are non-linear and in vitro theophylline metabolism by human liver microsomes does not obey monophasic kinetics. 9. The data suggest also that not only tobacco smoking but also chronic alcohol intake may influence theophylline elimination in man as ethanol induces CYP2E1.