A recombinant vaccinia virus (vCF13) containing and expressing the gene for human interleukin (IL)-2 (vCF13) was compared to a recombinant vaccinia transfection control strain containing the LacZ gene at the same insertion site (vTFCLZ-1) for their ability to augment the immunogenicity of murine colon adenocarcinoma cell lines CT26 and CA51 in Balb/c mice. Both recombinant vaccinia strains abolished tumorigenicity of 105 CT26 or CA51 tumor cells. vCF13-infected tumor cells that secreted human IL-2 as measured by both CTLL-2 and enzyme-linked immunosorbent assays induced delay in tumorigenesis when administered in two weekly subcutaneous injections 1 week prior to challenge with 105 uninfected tumor cells. Although three of five vCF13-CT26 immunized mice developed tumors by day 14 after challenge, intralesional injection of these tumors with vCF13 induced rapid regression, whereas all five tumors that developed in vTFCLZ-1 immunized mice showed no response to intralesional vTFCLZ-1. These preliminary results provide support for the potential utility of recombinant vaccinia/IL-2 in tumor immunotherapy.