Cloned Bacillus subtilis genes were used to transform B. subtilis auxotrophs. Intracellular forms of donor DNA were then analyzed by agarose gel electrophoresis and autoradiography. Only donor double stranded molecules were found in the cells and these were sensitive to restriction endonuclease in appropriate receipient cells. The same defined DNA segments were also used to cotransform mouse cells and a sensitive assay was developed to detect donor DNA sequences. Procaryotic carrier DNA was found to inhibit mammalian cell transformation. Addition of DNA to mammalian cells during S phase increased the frequency of stable transformation. The technology described here is sensitive and simple in analyzing intermediate steps in recombination in procaryotes and eucaryotes.