In the present work, we integrated a prepared bio-chip with ^99mTc labeling to improve the immunoassay for cancer biomarker protein detection. Efficient binding of a ^99mTc isotope-labeled secondary antibody to the biomarker protein pre-anchored to the bio-chip was followed by detection of γ rays from ^99mTc decay via a Gamma Well Counter. We show that alpha-fetoprotein (AFP), a biomarker of liver cancer, can be detected down to 10⁻¹⁸ M concentration in a buffer solution with good reproducibility. This ultra-high sensitivity is attributed to the aggregation of the ^99mTc isotope-labeled secondary antibody which greatly amplifies the γ rays emitted per AFP binding event. The tendency of the isotope-labeled secondary antibody to aggregate was indicated by dynamic light scattering studies on a rhenium/secondary antibody conjugate, a stable structural analogue of the ^99mTc conjugate. The successful zeptomolar-range detection of cancer biomarkers opens new avenues for early disease diagnoses via integrating bio-chips with isotope-labeling techniques.