The aim of this study was to develop a ¹⁸⁸Re-labeled porphyrin-based tumor-specific agent and to evaluate its biologic behavior, including tumor-regressing effectiveness, in mouse tumor models for possible use in achieving targeted cancer radiotherapy. ¹⁸⁸Re was obtained from an alumina-column-based ¹⁸⁸W-¹⁸⁸Re generator constructed in-house. The compound, 5,10,15,20-tetrakis [3,4-bis(carboxymethyleneoxy) phenyl] porphyrin, was synthesized and labeled with (ReO₄^-)-¹⁸⁸Re. ¹⁸⁸Re-labeled porphyrin complex was produced with a radiochemical purity of similar to 98% with reasonably good in vitro stability (>24 hours at 4 °C). Swiss mice bearing thymic lymphoma and fibrosarcoma were used as tumor models. The biodistribution studies revealed satisfactory tumor retention (2.07% ± 0.80% injected activity per g) with insignificant activities in blood (0.53%), liver (0.26%) and kidney (0.04%) at 24 hours. The radiolabeled conjugate treatment increased the average tumor-doubling time and decreased the average specific growth rate substantially in thymic lymphoma, compared to fibrosarcoma tumor. ¹⁸⁸Re-labeled 5,10,15,20-tetrakis[3,4 bis(carboxymethyleneoxy) phenyl] porphyrin has specific affinity toward the fibrosarcoma and thymic lymphoma tumors in mice. Thymic lymphoma was found to be more sensitive to the radionuclide complex, compared to fibrosarcoma. The ¹⁸⁸Re-labeled porphyrin complex showed promising results and warrants further investigations.