Carboxymethylated dextran (CMD)-coated magnetic iron oxide nanoparticles (MNPs) were synthesized using a co-precipitation method. Compared to neutral dextran coated MNPs, the CMD coating provides good dispersity and colloidal stability to the CMD-MNPs. In particular, the carboxyl groups on the CMD can be readily activated for covalent attachment of antibody molecules. The superparamagnetic property of the antibody-covered CMD-MNPs enables the captured antigen to be separated from the sample solution and CMD coating significantly reduces the nonspecific binding of the nanoparticles. Regeneration of the anti-BSA antibody-covered CMD-MNPs with NaOH does not significantly decrease the antibody activity, and the repeated magnetic separation and washing steps cause only small loss of the starting materials. The method was found to be highly reproducible (RSDs for BSA adsorption and desorption are between 0.78% and 5.1%). The anti-BSA antibody-covered CMD-MNPs possess good selectivity and are able to capture protein antigens from real samples.