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 188ReO4−. 188Re-labeled porphyrin complex was produced with a radiochemical purity of ∼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 rat...