Contemporary immunoassays commonly used in clinical diagnostics mostly utilize enzymes, such as horseradish peroxidase, for signal generation. Numerous research is dedicated to the development of artificial peroxidase-mimicking catalysts with lower cost, high activity, better operational stability, and tunable properties. Herein we synthesized hemin-loaded bovine serum albumin (BSA) nanoparticles and applied them as catalytic labels (nanozymes) in colorimetric immunoassay of anti-tetanus antibodies. Hemin is a key part of the peroxidase catalytic center, possessing peroxidase like-activity. Albumin nanoparticles were loaded with multiple hemin molecules and decorated with Streptococcal protein G. Resulting nanozymes possessed good colloidal stability and allowed for antibody detection in blood serum. The sensitivity of antibody detection was sufficient for the assessment of post-vaccination immunity.