In the lac operon, the existence of a secondary repressor binding site, inside Z gene, had been inferred from in vitro binding studies (Reznikoff et al., 1974; Gilbert et al., 1975). A series of deletions have been constructed from a lac transducing lambda bacteriophage. Some of those deleted bacteriophages have still the property of derepressing a chromosomal lac operon, even though they do not contain any more the lac operator. This phenomenon is an indication that the secondary repressor binding site is also active in vivo.