A water-soluble porphyrin, 5,10,15,20-tetrakis[3,4-bis(carboxymethyleneoxy)phenyl] porphyrin (T3,4BCPP), was successfully labeled with ^99 m^ Tc and biodistribution studies were performed in Wistar rats. Scintiimaging and in vivo distribution studies were also carried out in C ~6~-gliomas and mammary tumor-bearing animals using a gamma camera. Tumor-to-muscle (T/M) ratios were calculated and compared with those obtained with the known tumor-seeking radiopharmaceuticals ^99 m^ Tc (V)- DMSA (DMSA = dimercaptosuccinic acid), ^99 m^ Tc -citrate and ^201^ TlCl . In the case of C ~6~-gliomas, the ratios were 4.2, 2.2, 4.00 and 3.0; while in the case of C ~3~ H / J mammary tumor, the ratios were 9.4, 8.8, 8.1 and 8.5 for T3,4BCPP, ^99 m^ Tc ( V )- DMSA , ^99 m^ Tc -citrate and ^201^ TlCl , respectively. Similar studies were carried out in N-methyl-N-nitrosourea ( NMU )-induced mammary tumor animals and the T/M ratios obtained were 5.9, 2.0, 5.3 and 3.3 for T3,4BCPP, ^99 m^ Tc ( V )- DMSA , ^99 m^ Tc -citrate and ^201^ TlCl , respectively. The radiolabeled photosensitizer could perhaps be used to detect cancer non-invasively and could even prove useful in monitoring the progression/regression of tumors before, during, and after chemotherapy, radiation therapy or photodynamic therapy (PDT).