The potential for using gene therapy to treat a variety of disease states is growing rapidly. Many vector types and delivery systems have been developed that allow the optimization of protein production levels and kinetics for a given therapeutic gene product. In cases in which a transient, localized delivery of gene product is desired, any determination of the locale of transfected tissue by non-marker genes is problematic. We describe a technique by which the use of fluorescent microspheres can help in identifying potentially transfected tissue. Adenovirus containing the gene for β-galactosidase (β-gal) was mixed with fluorescent microspheres and injected into rat skeletal muscle and porcine myocardium. The injection sites could be visualized under ultraviolet light and correlated with β-gal enzyme expression. This method is simple, inexpensive and generally useful for in vivo gene transfer experiments.