The modified vaccinia virus Ankara (MVA) strain is a candidate vector for vaccination against pathogens and tumors, due to safety concerns and the proven ability of recombinants based on this vector to trigger protection against pathogens in animals. In this study we addressed the fate of the MVA vector in BALB/c mice after intraperitoneal inoculation in comparison with that of the replication-competent Western Reserve (WR) strain by measuring levels of expression of the reporter luciferase gene, the capability to infect target tissues from the site of inoculation, and the length of time of virus persistence. We evaluated the extent of humoral and cellular immune responses induced against the virus antigens and a recombinant product (beta-galactosidase). We found that MVA infects the same target tissues as the WR strain; surprisingly, within 6 h postinoculation the levels of expression of antigens were higher in tissues from MVA-infected mice than in tissues from mice infected with wild-type virus but at later times postinoculation were 2 to 4 log units higher in tissues from WR-infected mice. In spite of this, antibodies and cellular immune responses to viral vector antigens were considerably lower in MVA-inoculated mice than in WR virus-inoculated mice. In contrast, the cellular immune response to a foreign antigen expressed from MVA was similar to and even higher than that triggered by the recombinant WR virus. MVA elicited a Th1 type of immune response, and the main proinflammatory cytokines induced were interleukin-6 and tumor necrosis factor alpha. Our findings have defined the biological characteristics of MVA infection in tissues and the immune parameters activated in the course of virus infection. These results are of significance with respect to optimal use of MVA as a vaccine.