In gene therapy, a number of viruses are currently being used as vectors to provide transient expression of therapeutic proteins. A drawback of using free virus is that it gives a potent immune response, which reduces gene transfer and limits re-administration. An alternative delivery system is to encapsulate the virus in poly(lactide-co-glycolide) (PLG) microspheres prior to administration. A recombinant adenovirus (Ad) expressing green fluorescent protein (GFP) was used to test the transduction efficiency of Ad encapsulated in microspheres on target cells. The number of infected cells that expressed GFP was measured by flow cytometry. It was demonstrated that encapsulated viral vectors could successfully transduce target cells with encapsulation efficiencies up to 23% and that the level of transduction could be controlled by varying both the quantity of microspheres and the amount of Ad in the microspheres. High transduction efficiencies and its recognized biocompatibility make PLG-encapsulated Ad an attractive alternative to the use of free virus in gene therapy applications. The infectivity of Ad was found to be significantly influenced by the processing conditions and changes in environmental factors. Free Ad and encapsulated Ad were able to infect both E1 complimenting cells (HEK 293) and non-complimenting cells (A549), with the viral expression in HEK 293 cells being 2.1 times greater than for A549 cells.