We study the effect of functionalization of Carbon Nanotubes (CNTs) with a primary monoclonal mouse immunoglobin G (IgG) specific to the cell-surface receptors of breast cancer cells, and secondary polyclonal goat ant- mouse IgG. The CNTs, in solution with a surfactant (sodium dodecyl benzene sulfonate) were labeled with dihexyloxacarbocyanine iodide (DiOC6), a fluorescent dye, in order to view them with fluorescently labeled antibodies through confocal microscopy. Colocalization for CNTs in combination with the primary antibody conjugated to the secondary was determined to be 90%, whereas CNTs in combination with the secondary antibody and polyethylene glycol (PEG), a polymer used to block CNTs from proteins binding to their surface, was found to be very minimal (0.5%). Preliminary studies on the electrical measurements of the primary mouse IgG incubated with CNTs show a decrease in conductance compared to that of bare CNT field effect transistors (CNTFETs). This observed change in conductance, can eventually be amplified and utilized in applications leading to a full-fledged breast cancer detection system in the future. © 2004 IEEE.