Surgical ablations revealed different dependencies on the injection site tissue for i.m. needle and epidermal gene gun DNA immunizations. Excision of an injected muscle bundle within 10 min of DNA inoculation did not affect the magnitude or longevity of Ag-specific Ab responses. By contrast, biopsy of the skin target site up to 24 h after gene gun bombardment completely abrogated the Ab response in the majority of mice. The form of a DNA-expressed Ag (membrane bound, secreted, or intracellular) did not affect the temporal requirements for the skin and muscle target sites. Skin cells, but not muscle cells, were also required for DNA-induced CTL. We conclude that transfected cells in gene gun-bombarded skin, but not needle-injected muscle, play a central role in DNA-initiated Ab and CTL responses.