Approval for the use of the vaccine in the Phase I trial was obtained from the FDA under IND #9706. In pre-clinical studies, our group has demonstrated that the Ad/PSA vaccine was able to induce stronger anti -PSA immune responses than other viral PSA vaccines. These include vaccinia viruses, both replication competent and replication deficient, and to a canarypox vaccine (Table 1) , The frequency of PSA-specific CD8+ cells T cells generated by the Ad/PSA vaccine was greater than were generated by any of the other vaccines tested.. In addition to the superior immunizing property of the Ad/PSA, the incorporation of Gelfoam, a collagen matrix (section 1.3) , has been shown in pre-clinical studies to enhance the ability of the vaccine to induce strong anti - PSA immune responses (8). Lastly, immunization of mice with Ad/PSA in matrix can induce anti - PSA responses even in the presence of high titer anti -adenovirus antibodies (8). This latter finding is important in light of the fact that most humans have pre -existing levels of anti -adenovirus antibodies as a result of prior natural exposure to the virus. Table 1 Effector Cell Frequency Analysis (ELISPOT) Vaccine Virus Frequency of PSA -Specific CD8+ T Cells Ad/PSA * Replication deficient adenovirus 1/455 Prostvac Replication competent vaccinia 1/2028 NYVAC/PSA Replication deficien t vaccinia 1/3597 ALVAC/PSA Canarypox 1/35,714 1.3 Gelfoam® Matrix Gelfoam (Pharmacia & Upjohn Company, Kalamazoo, MI) is a medical device intended for application to bleeding surfaces as a hemostatic agent. It is a water -insoluble, off -white, non- elastic, porous, pliable product prepared from purified pork skin.