The evidence for horizontal gene transfer from genetically modified (GM) crops (i.e. maize) to bacteria and mammalian cells is reviewed. The conclusion is that while horizontal gene transfer can and has occurred, such events are rare. However, even rare events may have an ecological impact, and thus the genes encoded by DNA introduced into a GM plant should be the focus of biosafety considerations. In the case of antibiotic resistance markers, the chances of increasing the fitness of any bacteria acquiring the genes from a GM plant is remote. There is also no known risk associated with the remote possibility that mammalian cells could be transformed with these genes and express the proteins.