Leftward transcription in the region of the left attachment site (att) has been found to be required for the excision of λ prophage, when a helper provides all the necessary excision proteins (Davies et al., 1972). That observation has allowed the discovery of another component in λ site-specific recombination. This additional component is revealed by deleting a part of the b2 region of the helper. It is asserted that the newly discovered function is an inhibitor of site-specific recombination. The effect of the two components, local transcription and the b2 inhibitor, has been further studied in the excision of prophages flanked by other than the normal pair of atts.