Ongoing studies involves constructing and evaluating the safety and immunogenicity of recombinant pox viruses expressing human tumor associated antigens. These include carcinoembryonic antigen (CEA) and prostate specific antigen (PSA). We demonstrated that CEA could be used as a target for active specific immunotherapy. Immunization of mice with a recombinant vaccinia virus expressing CEA (rV-CEA) was able to elicit anti-tumor activity. Other tumor associated antigens that represent tumor targets are members of the human mucin gene family such as MUC1 and MUC2 in which gene expression is upregulated in a variety of epithelial cancers such as breast, colorectal, ovarian, and lung. The mucin gene family members are complex in their genetic structure. They contain 100-150 DNA segments of highly repeated nucleotide regions of 60-120 base pairs (VNTR variable number tandem repeats). VNTRs are unstable in vaccinia viruses. A stable recombinant vaccinia virus expressing MUC1 was constructed by truncating the VNTR region to contain only 10 VNTRs. This vaccine was shown to elicit specific MUC1 humoral, cell mediated and anti-tumor responses in a metastatic murine tumor model. Various costimulatory signals delivered through T cell surface molecules are required for the activation of naive T cells by antigen bearing target cells. The expression of the B7 gene family as well as other costimulatory molecules such as CD70, LFA3, and ICAM-1 have been shown to be an important component for the induction and maintenance of anti-tumor responses in experimental models. We have constructed and characterized recombinant vaccinia viruses containing B7.1 and ICAM-1. Tumor cells infected with rV-B7.1 resulted in no tumor growth in the animals while tumor cells infected with wild type vaccinia virus lead to the death of the host. These studies demonstrated the utility and ease of using recombinant vaccinia viruses to deliver costimulatory molecules to tumor cells for potential gene therapy and recombinant approaches to cancer immunotherapy. We have also demonstrated the utility of admixing these recombinant costimulatory vaccinia viruses with rV-CEA and rV-MUC1 to enhance specific T-cell responses and antitumor immunity in murine tumor models. Improvement of T-cell immunogenicity and specificity of tumor associated antigen vaccines has been accomplished by constructing recombinant vaccinia viruses containing minimal determinants of an immunodominant epitope of tumor associated antigens. Second generation vaccines are being developed using nonreplicating pox viruses such as avipox, fowlpox, and MVA (modified vaccinia ankra).