Due to the anti-proliferative properties of cobalt-thiosemicarbazone complexes, the production of [55Co](III)-bis-(2-acetylpyridine thiosemicarbazone) ([55Co](III)[APTS]2) was investigated. Co-55 (T1/2=17.53 h) was produced by 150 μA irradiation of a natural nickel target by 15 MeV protons. The 55Co was separated from the irradiated target material using a two-step method with a radiochemical yield of >95% followed by radionuclidic and chemical purity control. [55Co](III)chloride was mixed with 2-acetylpyridine thiosemicarbazone for 30 min at room temperature to yield [55Co](III)[APTS]2 (radiochemical purity > 98% shown by RTLC/HPLC). A specific activity of about 10–20 Ci/mmol was obtained. The final solution was diluted in normal saline to 5% ethanolic solution for biological evaluation. The stability of the final product was checked in the absence and presence of human serum at 37°C to 24 h. The partition co-efficient of the final complex at the pH of 7 was 1.00±0.08. A significant tumor accumulation (%ID/g; 3.5%) was observed in tumoral tissue 21 h post injection in fibrosarcoma-bearing mice by biodistribution studies. Co-incidence imaging also demonstrated tumor uptake from 21–35 h however at 35 h tumor uptake is more specific and significant.