The biolistics process uses high velocity microprojectiles to carry foreign DNA into cells. Though biolistics has already proven to be useful fo r a wide range of species, improvements are still needed, and many of the factors which affect transformation efficiency have not been defined. In our experiments, cell suspensions of Nicotiana tabacum (NT1 line) were used as a model to identify these factors. The most critical factors for high efficiency transformation were: cell age, microprojectile type & size, DNA construct, osmoticum in the bombardment medium, use of a new helium-driven biolistic device, and the handling and growth environment of the cells after bombardment. By optimizing these factors, an average of 7,000 transiently expressing GUS cells and 800 kanamycin resistant colonies were obtained per bombarded plate. The high efficiency and rapid results (2 days transient/4 weeks stable) of the NT1 model system make it useful for cell biology studies and for testing DNA constructs.