Assessing the activity of a particular drug-metabolizing enzyme (i.e., its phenotypic status) has practical implications in the fields of therapeutic drug monitoring and clinical toxicology as well as for the biological monitoring of environmental and/or occupational exposure to chemical agents. This report describes a novel, convenient, and rapid analytical method for phenotyping cytochrome P4502E1 (CYP2E1), an important drug-metabolizing enzyme in industrial toxicology (1). The measurement of CYP2E1 expression also has applications for monitoring alcoholic patients (2). Although the most accurate assessment of CYP2E1 activity is based on direct hepatic measurement, which requires a liver biopsy, alternative noninvasive approaches have been developed. Until now, the “gold standard” method for CYP2E1 phenotyping has used a probe drug, chlorzoxazone (CZX), administered orally (500 or 250 mg) to fasting individuals and metabolized to 6-hydroxychlorzoxazone (HCZX) by CYP2E1. Various CZX pharmacokinetic measures have been proposed to reflect CYP2E1 activity, as reviewed recently (3): plasma CZX t 1/2β, HCZX renal excretion, CZX oral clearance, fractional clearance to HCZX, and HCZX area under the concentration curve (AUC). However, these procedures are time-consuming, laborious (e.g., urine collection), and require subject compliance and catheterization. Furthermore, recent in vitro studies suggest that CYP1A1, CYP1A2, and possibly CYP3A are involved in CZX 6-hydroxylation, limiting the specificity of CZX for CYP2E1 phenotyping (3). CYP2E1 phenotyping for screening or follow-up of large numbers of individuals, in epidemiological studies, or even as an adjunct to improve biological monitoring programs requires the implementation of more convenient methods. Circulating lymphocytes have been used as surrogates because CYP2E1 expression in these cells appears to parallel the concentration of the hepatic enzyme, suggesting that both cell types are influenced by the same factors, including xenobiotics and physiological state (4). In a recent report involving alcoholics and controls (2), pharmacokinetic parameters for CZX hydroxylation (CZX clearance …