Novel octylthio-containing asymmetrically substituted Zn(II) phthalocyanine (Zn(II) Pc1) and a symmetric derivative (Zn(II) Pc2) have been prepared to investigate the biological potential and ability to photosensitize singlet oxygen for photodynamic therapy applications. In this study, the singlet oxygen generation potential and in vitro photodynamic activities of these compounds have been tested. Both ZnPcs reveal to be very efficient singlet oxygen generators and promising PSs for PDT applications. In vitro PDT activities of the compounds were evaluated in EMT-6 murine mammary carcinoma and HeLa human cervix carcinoma cell lines. Moreover, Zn(II) Pc1 displayed the phototoxic effects in the mammary cancer cell line (6.25 mu M concentration at 30 J/cm(2) light dose and 12.5 mu M concentration at 20 J/cm(2) light dose), while Zn(II) Pc2 did not show any phototoxic effects both in two cell lines. Zn (II) Pcs were radiolabeled with I-131 in high yields. Biodistribution studies revealed that the radiolabeled Zn(II) Pc1 showed significant uptake in l. intestine, pancreas, brain, and ovary, while Zn(II) Pc2 has significant uptake in ovary and pancreas in normal rats. Hence, these Pcs derivatives could be promising candidate for tumor nuclear imaging.