The transducing phage λh80dlac carries the lac operator, whereas wild-type λh80 does not. We find that in high salt (0.18 M KCl), ultraviolet radiation causes the formation of a very stable complex between repressor and 5-bromodeoxyuridine (BrdU)-substituted λh80dlac but not to BrdU-λh80 DNA. Studies with inducers of the lac operon confirm the specificity of attachment. In low slat (0.01 M KCl), ultraviolet radiation will also attach repressor nonspecifically to BrdU-λh80 DNA. The stability of the complex suggests that covalent bonds are formed. We also report that another regulatory protein, the catabolite gene activator protein, can be attached similarly to DNA.