Safety concerns about introducing genetically engineered cells into the body have prevented their use in medical treatments. To solve this problem, we prepared polymeric membrane artificial cells (semipermeable microcapsules) containing genetically engineered live cells from the bacteria Escherichia coli DH5. When given orally, the cells remain at all times in the microcapsules and are finally excreted in the stool. During their passage through the intestine, small molecules like urea diffuse rapidly into the microcapsules and are acted on by the genetically engineered cells. This lowers the high plasma urea level to normal in uremic rats with induced kidney failure, and has exciting implications for the use of this and many other types of genetically engineered cells in a number of medical applications.