We have introduced the firefly luciferase gene of Photinus pyralis into the vaccinia virus genome. This gene is expressed in a coordinate fashion during virus infection. Luminescence produced by the action of luciferase [Photinus-luciferin:oxygen 4-oxidoreductase(decarboxylating, ATP-hydrolyzing), EC 1.13.12.7] was easily detectable in infected cells in culture as well as in cells of tissues of infected mice. The limits of detection were about one infected cell in a background of a million noninfected cells. The luciferase assay was about 1000-fold more sensitive than that of beta-galactosidase. Our findings show that the luciferase assay can be conveniently used to follow viral gene expression and virus dissemination both in cell cultures and in tissues of infected animals.