Besides hepatic P450 (cytochrome P450) metabolism, there is increasing interest in the possibility of intratumoral activation of oxazaphosphorines by P450. Therefore, we investigated the expression of P450 (CYP2C8, CYP2C9, CYP2C18, and CYP2C19) by RT (reverse transcriptase)-polymerase chain reaction (PCR) and of CYP2C9 by Western blotting in 10 different breast tumor samples. Since P450 may be down regulated by interleukin (IL) IL-6, the receptor (R) for IL-6 was analyzed by RT-PCR and IL-6 in supernatants was calculated from ELISA data. None of the breast tumors was positive for CYP2C18 and CYP2C19 mRNA, whereas CYP2C8 and CYP2C9 were detected in all 10 breast tumors. Correspondingly, all breast tumors tested (9 of 10) revealed low, but nevertheless positive, staining of the CYP2C9 protein. All 10 samples were positive for the IL-6 receptor mRNA. ELISA measurement of IL-6 cytokine in supernatants revealed that all measured samples (8 of 10) were producing IL-6, the amounts ranging from 0.004 to 3.1 ng/g(tumor tissue). In summary, we have demonstrated that tumors of the breast express two out of four members of the CYP2C family, indicating that activation of such prodrugs as oxazaphosphorines may take place intratumorally. The presence of the IL-6 receptor and of IL-6 cytokine, which is produced in an autocrine manner, opens up the possibility that the well-known down regulating effect of IL-6 also takes place in breast tumors and might explain the weak or even absent expression of different CYP2C members.