A magnetoelastic immunosensor for detection of staphylococcal enterotoxin B (SEB) is described. The magnetoelastic sensor is a newly developed mass/elasticity-based transducer of high sensitivity having a material cost of approximately $0.001/sensor. Affinity-purified rabbit anti-SEB antibody was covalently immobilized on magnetoelastic sensors, of dimensions 6 mm x 2 mm x 28 microm. The affinity reaction of biotin-avidin and biocatalytic precipitation are used to amplify antigen-antibody binding events on the sensor surface. Horseradish peroxidase (HRP) and alkaline phosphatase were examined as the labeled enzymes to induce biocatalytic precipitation. The alkaline phosphatase substrate, 5-bromo-4-chloro-3-indolyl phosphate (BCIP) produces a dimer, which binds tightly to the sensor surface, inducing a change in sensor resonance frequency. The biosensor demonstrates a linear shift in resonance frequency with staphylococcal enterotoxin B concentration between 0.5 and 5 ng/ml, with a detection limit of 0.5 ng/ml.