To investigate the targeting ability of the PC5-2 phage in vivo, we injected phages into the tail vein of mice bearing CL1-5-derived tumors and then recovered them after perfusion. We determined the titers of the phage in tumor masses and normal control organs (brain, heart and lungs) [26, 30] . PC5-2 showed specific homing to tumor masses with concentrations 15-fold higher than its concentration in the control organs ( Fig. 2A) . Control helper phages did not show any specific targeting to tumor tissues ( Fig. 2A) .