Cultured primary porcine urinary bladder epithelial cells (PUBEC) represent an adequate and easy to handle in vitro system for studies of urothelial toxicity. PUBEC maintain in vivo-like metabolic activities and physiological functions. They express inducible cytochrome P4501A isoenzymes, which are of particular relevance, since they contribute to activation of bladder carcinogens. A possible drawback of PUBEC is their isolation from common domestic pigs that do not represent an inbred strain. In order to further establish PUBEC as a standard in vitro toxicity test system we analysed possible interindividual differences in CYP1A1 inducibility. Interestingly, we observed by flow cytometry that PUBEC obtained from individual pigs consist of two distinct subpopulations with inducible and non-inducible cells. A strong, concentration-dependent CYP1A1 induction was observed in the responsive subpopulation when incubated with benzo[a]pyrene (B[a]P) in a concentration range between 1 and 10 muM. In contrast, no CYP1A1 induction was obtained in the non-responsive subpopulation up to the highest tested concentrations of 100 muM. The fraction of responsive cells showed large interindividual differences ranging from 10 to 65% of the total cell number. For practical purposes it might be reasonable to analyse pools of PUBEC from five pigs which substantially reduce batch to batch variability. In conclusion, we have identified two functionally distinct subpopulations of urinary bladder epithelial cells. It will be interesting to study whether the CYP1A inducible subtype is more susceptible to bladder carcinogens.