We investigated the biocompatibility, specificity, and activity of a ligand-receptor-protein system covalently bound to oxidized single-walled carbon nanotubes (SWNTs) as a model proof-of-concept for employing such SWNTs as biosensors. SWNTs were functionalized under ambient conditions with either the Knob protein domain from adenovirus serotype 12 (Ad 12 Knob) or its human cellular receptor, the CAR protein, via diimide-activated amidation. We confirmed the biological activity of Knob protein immobilized on the nanotube surfaces by using its labeled conjugate antibody and evaluated the activity and specificity of bound CAR on SWNTs, first, in the presence of fluorescently labeled Knob, which interacts specifically with CAR, and second, with a negative control protein, YieF, which is not recognized by biologically active CAR proteins. In addition, current-gate voltage (I-V(g)) measurements on a dozen nanotube devices explored the effect of protein binding on the intrinsic electronic properties of the SWNTs, and also demonstrated the devices' high sensitivity in detecting protein activity. All data showed that both Knob and CAR immobilized on SWNT surfaces fully retained their biological activities, suggesting that SWNT-CAR complexes can serve as biosensors for detecting environmental adenoviruses.