Introduction: Porphyrin and its derivatives exhibit inherent affinity for localization in tumors. Hence, porphyrin derivatives radiolabeled with suitable therapeutic radionuclides could be envisaged as potential agents for targeted tumor therapy. In this direction, a water-soluble porphyrin derivative, viz., 5,10,15,20-tetrakis[4-carboxymethyleneoxyphenyl] porphyrin was synthesized in-house and radiolabeled with ¹⁷⁷Lu with an aim to prepare an agent for targeted tumor therapy. ¹⁷⁷Lu is an attractive radionuclide for the development of targeted radiotherapeutic agents owing to its suitable decay characteristics [T_1/2=6.73 d, E-b(max)=0.49 MeV, E_g=208 keV (11%)], comparatively longer half-life and ease of production with high specific activity. Methods: ¹⁷⁷Lu was produced by irradiation of enriched Lu₂O₃ (64.3% ¹⁷⁶Lu) at a thermal neutron flux of 1x10¹⁴ n/cm².s for 14 d. The porphyrin was coupled to a suitable chelator, namely, p-aminobenzy1-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid for complexation with ¹⁷⁷Lu. The radiolabeling was achieved by incubating 50 mu g of the conjugate with (LuCl₃)-¹⁷⁷Lu (200 ng Lu) in acetate buffer (pH ~ 5) at 50°C for 1 h. The radiolabeled conjugate was characterized by high-performance liquid chromatography and its biological efficacy was studied in Swiss mice bearing fibrosarcoma tumors. Results: ¹⁷⁷Lu was obtained with a specific activity of similar to 550 TBq/g and radionuclidic purity of 99.98%. The ¹⁷⁷Lu labeled porphyrin conjugate was obtained with 99% radiochemical purity and it exhibited good in vitro stability. Biodistribution studies revealed good tumor uptake (2.01% IA/g) within 3 h post injection (p.i.) with >94% injected activity exhibiting renal clearance. No significant accumulation of activity was observed in any of the vital organs/tissue. The tumor/blood and tumor/muscle ratios were 2.89 and 16.80, respectively, at 3 h p.i. and further increased till 2 days p.i. up to which the studies continued. Serial scintigraphic images recorded using a gamma camera exhibited significant accumulation of activity in tumor over background at 3 days p.i., and the activity was observed to be retained in the tumor till 14 d. Preliminary efficacy studies carried out in Swiss mice bearing fibrosarcoma tumors showed significant regression of the tumor growth in the treated animals. Conclusion: Bioevaluation and preliminary tumor regression studies provide supportive evidences toward the possible potential of the ¹⁷⁷Lu-labeled porphyrin for targeted tumor therapy.