Newborn infants are susceptible to bleeding disorders caused by a vitamin K deficiency, so called 'haemorrhagic disease of the newborn' (HDN). These bleedings often occur in infants after medication of the mother with antiepileptics, such as phenobarbital or phenytoin. It has been suggested that an increase in the late type of HDN in exclusively breast-fed infants might be related to the presence of cytochrome P450-inducing polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in human milk. In order to study this possible mechanistic relationship 5-week-old, germfree, female WAG/Rij-rats were exposed to a single oral dose of either 1 microgram 2,3,7,8-tetrachlorodibenzo-p-dioxin/kg body weight (TCDD) or 30 mg 2,2',4,4',5,5'-hexachlorobiphenyl/kg body weight (HxCB), representing cytochrome P-450 1A (3-methylcholanthrene type) and 2B (phenobarbital type) inducers. During the experiment blood coagulation time from each rat was measured. Also, hepatic 7-ethoxy-(EROD) and 7-pentoxyresorufin O-dealkylating (PROD) activities and total cytochrome P450 content were measured. Blood coagulation time (Thrombotest) in the HxCB-treated rats was significantly prolonged and positively correlated to PROD activity and total P450 content. No clear effect of TCDD on coagulation time could be observed under these experimental conditions. These results suggest involvement of P450 2B isoenzymes in vitamin K metabolism.