The tissue distribution of PC5-2 was also studied using immunostaining. We injected SCID mice bearing NSCLC xenografts with PC5-2 and then removed and fixed the tumor and control organs for localization of the phage particles. PC5-2 was found to localize in tumor tissues ( Fig. 2Cd) . At a higher magnification, the immunoreactivity of the phage was detected on the plasma membrane with some diffusion in the surrounding cytoplasm of tumor cells (Fig. 2Ce ). There was no reaction product detected on normal organs such as brain, heart and lung tissues ( Fig. 2C a-c), nor on tumor tissues treated by the control phage (Fig. 2Ci) . The specific targeting of PC5-2 to the NSCLC xenograft was inhibited by the synthetic peptide SP5-2 in the in vivo homing experiment (Fig. 2Cj ).