The tumor-homing ability of PC5-2 was further confirmed by a peptide competitive inhibition experiment, in which synthetic peptide SP5-2, injected together with PC5-2, markedly inhibited the recovery of the phage from tumor masses (Fig. 2B) . One hundred micrograms of SP5-2 inhibited 92% of PC5-2 binding to tumor masses, but the same concentration of a control peptide had no such inhibitory effect (Fig. 2B) .