DNA-coated Au particles were accelerated by pressurized He gas to supersonic velocities for introduction of a gene into cells. Experimental and theoretical analyses both revealed a heterogeneous distribution of the particles per shot (1 mg Au = 2.4 x 10(7) particles with 2 microg [32P] DNA = 2.5 x 10(11) moles). For introduction of genes into the liver of living rats, the best results were obtained with a newly developed hand-held gene delivery system. The beta-galactosidase gene introduced into rat liver with Au particles by He at 250 psi was expressed (1.2 microunits/microg protein) in a limited area of the liver surface (8 x 8 mm, depth 0.5 mm). When the same gene gun was used on a monolayer of cultured COS7 cells (about 5 microm thick), cells were lost in the central area of heavy bombardment. Cell death caused by influx of Ca2+ was prevented by the use of the cytosol-type culture medium.