[Co(phen)3](3+)/multi-walled carbon nanotubes modified graphite electrode ([Co(phen)(3)](3+)/ MWNT/C) has been developed to monitor quantitatively 6-mercaptopurine (6-MP) concentration and to study the effect of rabbit blood as disturbance agents on the determination. Multi-walled carbon nanotubes (MWNT) are used for efficient immobilization of [Co(phen)3](3+). The prepared [Co(phen)(3)](3+)/MWNT/C electrode exhibits an excellent redox activity. 6-MP is detected by monitoring the reductive current values of free [Co(phen)(3)](3+) or the [Co(phen)(3)](3+)-6-MP complex generated by the interaction of [Co(phen)(3)](3+) on the MWNT modified graphite electrode with 6-MP. These electrochemical signals are found to be sensitive enough to monitor 6-MP concentration changes, and the [Co(phen)(3)](3+)/ MWNT/C electrode has also been applied to the evaluation of 6-MP in the presence of rabbit blood. (C) 2008 Elsevier B.V. All rights reserved.