Résumé : Classical inhibitors of human cytochrome P450 3A4 activity, such as ketoconazol and quercetin, are tested to prove the efficiency of a new metabolisation model using living entire cells. Grapefruit juice is a well-known potent inhibitor of cytochrome P450 3A4 activity. With regard to the clinical relevance of grapefruit juice-drug interactions, an investigation of other common juices is undertaken with this in vitro model. The CYP3A4 activity is measured by the formation of the 6β-hydroxytestosterone, which is quantified by an isocratic high performance liquid chromatography. It is demonstrated for the first time that lemon juice significantly inhibits by 60 ± 3% the CYP3A4-mediated oxidation. Grapefruit juice inhibits this activity by 82 ± 4%. The mechanism of lemon juice inhibition is competitive, whereas it is mixed for grapefruit juice. These results suggest that our in vitro model combined with our analytical method is applicable for the investigation of the inhibition of CYP3A4 not only by chemical inhibitors but also by natural food products. © 2001 Elsevier Science B.V.