The development of an efficient transformation system is a prerequisite for the molecular analysis of gene expression in plants. In crop plants, this development has been hindered by difficulties encountered both in whole plant regeneration from protoplasts and in the general insusceptibility of monocots to Agrobacterium-mediated transformation. We have circumvented these difficulties by transferring foreign genes directly into the intact cells (with cell walls) of three important crop plants including rice, wheat and soybean by a particle bombardment device. Oryza sativa and Triticum monococcum cells were bombarded with accelerated tungsten particles coated with plasmids containing a β-glucuronidase gene as the reporter. Blue transformed cells were detected in an in situ enzyme assay. The number of blue cells was next used as a convenient criterion to study several factors affecting gene transfer efficiency. After optimal conditions were defined, gene transfer into intact cells of O. sativa, T. monococcum and Glycine max was successfully carried out with chloramphenicol acetyltransferase (CAT) gene as the reporter.