Summary Tungsten and gold particles, coated with plasmid DNA harboring the β‐glucuronidase (GUS) and neomycin phosphotransferase II (npt‐II) genes, were delivered into tobacco primary leaves and suspension‐cultured cells of maize using the helium particle inflow gun. Cell viability and particle localization were determined 1 and 2 days after bombardment. Of the counted particles, 7–10% penetrated into or through the epidermis. Blue spots on tobacco leaves appeared as a blue area around a single, densely stained particle‐containing central cell. DNA‐coated gold particles provoked smaller spots with less diffusion and gave rise to more individual events than tungsten particles. In more than 90% of the GUS‐positive epidermal and mesophyll cells, a particle was detectable within their nucleus. Two days after bombardment, viability had decreased to 1–2% in particle‐containing cells. Penetration of a cell by a particle was accompanied by callose formation in the wound area. Dead suspension culture cells of maize without callose formation but containing particles were detected just 1 h post‐bombardment. Living cells with callose spots appeared more frequently after bombardment with tungsten than gold. As in tobacco, GUS expression was limited to those cells containing a particle in their nucleus, and the number of particle‐containing, viable cells was low after 48 h. The frequency of stable expression events was compared to the number of surviving tobacco leaf cells. On average, four kanamycin‐resistant calli or plantlets were recovered per bombarded dish, of which approximately 50% were also GUS‐positive. This corresponds to a stable‐to‐transient ratio of approximately 0.8%, and is similar to the number of particle‐containing cells surviving after 48 h.

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