The characterization of metabolic activity of intestinal cells in culture may be of particular interest for the study of the effects of food additives and contaminants which, ingested through the diet, are absorbed primarily by the gut mucosa. IEC-17 cell line is derived from the intestine of newborn rat and has been shown to be competent for the metabolism of xenobiotics (Quaroni and Isselbacher, 1981), and to keep some degree of differentiation also in in vitro conditions. Our studies indicate that the xenobiotic metabolic activity of this cell line, as shown by measurement of 7-ethoxycoumarin (7-EC), can be induced by beta-naphthoflavone (NF) and not by phenobarbital (PB) and only at late subculturing stages, suggesting these cells undergo some kind of maturation in vitro. Induction of xenobiotic metabolism by beta-naphthoflavone seems also to elicit the toxicity of 9,10-dimethyl-1,2-benzathracene (DBA) but not that one of cyclophosphamide (CPA).