Ovalbumin-related protein X (OVAX) and ovalbumin are two very close ovalbumin-related serpins. As primary data on OVAX remain recent, information about possible cross-reaction of available antiovalbumin antibodies with OVAX is still missing. Using labeled purified OVAX and dot ligand blotting, we identified 49 recombinant dromedary antiovalbumin single domain antibody (sdAb) fragments that were unable to bind OVAX. Discrimination between OVAX and ovalbumin was confirmed for two of the corresponding sdAb fragments by surface plasmon resonance and Western ligand blotting (WLB) characterizations. Furthermore, they were covalently linked to Sepharose and used as an affinity matrix for ovalbumin depletion. At least 90% of the original ovalbumin was eliminated from the allantoic fluid of 14 day old chicken embryo in one step. These sdAb fragments, which bind ovalbumin with nanomolar affinity, should also contribute to a better characterization of ovalbumin preparations.