We describe the structure, activity, and stability of enzymes covalently attached to single-walled carbon nanotubes (SWNTs). Conjugates of SWNTs with three functionally unrelated enzymeshorseradish peroxidase, subtilisin Carlsberg, and chicken egg white lysozymewere found to be soluble in aqueous solutions. Furthermore, characterization of the secondary and tertiary structure of the immobilized proteins by circular dichroism and fluorescence spectroscopies, respectively, and determination of enzyme kinetics revealed that the enzymes retained a high fraction of their native structure and activity upon attachment to SWNTs. The SWNT−enzyme conjugates were also more stable in guanidine hydrochloride (GdnHCl) and at elevated temperatures relative to their solution counterparts. Thus, these protein conjugates represent novel preparations that possess the attributes of both soluble enzymeshigh activity and low diffusional resistanceand immobilized enzymeshigh stabilitymaking them attractive choices for applications ranging from diagnostics and sensing to drug delivery.