We report the activity, stability, and reusability of enzyme-carbon nanotube conjugates in aqueous solutions. A variety of functionally unrelated enzymes were covalently attached to oxidized single- and multi-walled carbon nanotubes (SWNTs and MWNTs). These conjugates were soluble in aqueous buffer, retained a high fraction of their native activity; and the intrinsic high length of the nanotubes led to facile filtration for reuse. The nanotube-enzyme conjugates were also more stable at elevated temperatures relative to their solution phase counterparts. Furthermore, the high surface area of SWNTs afforded high enzyme loadings, which allowed for a detailed characterization of the secondary and tertiary structure of the immobilized proteins by circular dichroism and fluorescence spectroscopies, respectively. These enzyme conjugates represent novel preparations that possess the virtues of both soluble and immobilized enzymes, thus providing a unique combination of useful attributes such as low diffusional resistance, high activity and stability, and reusability making them attractive choices for applications ranging from diagnostics and sensing to drug delivery.