The potential hazards of recombinant DNA research have focused attention on the development of a strain of bacteria that will not survive outside the laboratory. A genetically engineered strain of Escherichia coli K-12 has been developed that not only has several growth requirements (including diaminopimelic acid), but also exhibits high sensitivity to ultraviolet radiation, bile salts, detergents, antibiotics and temperature. This debilitated strain has been designated E. coli χ1776 and has been widely recommended for use in recombinant DNA research1. So far only conventional animals have been used to test the ability of a genetically engineered bacterium to replicate in vivo. We postulated that lack of competition from the normal bacterial flora, due to disease state or to antibiotic therapy, might increase susceptibility to colonisation by E. coli χ1776. The germ-free animal represents an extreme state in which resistance to bacterial colonisation is decreased by lack of indigenous microbial competition and by an underdeveloped immune system 2. We therefore attempted to colonise germ-free rats and mice (maintained at the University of Wisconsin, Gnotobiotic Laboratory) with E. coli χ1776. Each of several attempts to colonise the gastrointestinal tract was unsuccessful; viable organisms could not be cultured from either fecal material or tissue samples for up to 2 months after inoculation.