This paper presents real-time detection of breast cancer cells through label-free protein biosensors based oil antibody-conjugated carbon nanotube field effect transistors (CNTFETs). Both insulin-like growth factor 1 receptor (IGF1R) specific and nonspecific monoclonal antibodies (mAbs) reduced the electrical conductance of the CNTFET devices. Subsequent application of human BT474 and MCF7 breast cancer cells resulted in an increase in the conductance only for the IGF1R-specific mAb-CNTFET devices. Furthermore, the MCF7 breast cancer cells produced a greater increase in device conductance compared to BT474 breast cancer cells, This could be attributed to the larger number of IGF1 receptors in MCF7 breast cancer cells compared to BT474 cancer cells. Nonspecific mAb-CNTFETs did not yield any change in the Conductance of the devices due to application of BT474 and MCF7 breast cancer cells. These results therefore imply that specific binding between IGF1 receptor and its specific mAb results in a dramatic change in electrical conductance of CNTFET devices. Consequently, mAb-CNTFETs are very promising biosensor candidates to detect circulating cancer cells in blood samples. (C) 2008 Elsevier B.V. All rights reserved.