Carbon nanotubes (CNTs) are remarkable solid state nanomaterials due to their unique electrical and mechanical properties. The electronic properties of nanotubes combined with biological molecules such as proteins could make miniature devices for biological sensing applications. In this paper, the interaction of single wall carbon nanotubes (SWNTs) with antibodies are presented. This approach is used towards developing a biosensor for breast cancer detection, by functionalizing the CNTs with antibodies that are specific to cell surface receptors of breast cancer cells. The degree of binding of antibodies on CNTs was found to be 67-88% by confocal microscopy. The key to achieve such high degree of functionalization is due to the separation of CNTs using surfactants that leads to a high surface area to volume ratio and higher number of active sites for charge transfer that enhances binding. Further, van der Waals forces and the charges on the antibodies and SWNTs are manipulated to achieve specificity of binding. © 2004 IEEE.