Human CD34+ cells (haematopoietic stem and progenitor cells separated from peripheral blood) were shown to express CYP2E1 protein by Western blotting. For the first time, the specific CYP2E1 activity (chlorzoxazone 6-hydroxylation) was also detected. No CYP3A4 protein neither the CYP3A-specific nifedipine oxidase activity were detectable. The results obtained indicate differences in content of active CYP proteins in populations of stem and progenitor cells from different species as the CYP3A2 (a rat form similar to human CYP3A4) was shown to be expressed in bone marrow derived cells by RT-PCR (Avital et al. 2001).