The requirement for nucleotide hydrolysis in the DNA repair mechanism of the Escherichia coli UvrABC protein complex has been analyzed. The DNA-activated UvrAB ATPase activity is part of a helicase activity exhibited by the UvrAB protein complex. The helicase acts only on short duplexes and, therefore, is unlike other helicases such as those involved in DNA replication that unwind long duplexes. The strand displacement activity occurs in the 5'----3' direction and requires either ATP or dATP. The helicase activity is inhibited by UV photoproducts. The absence of this activity in a complex formed with proteolyzed UvrB (UvrB*), a complex also deficient in the endonuclease activity, suggests that this activity is important in the repair mechanism. The UvrAB protein complex may remain bound to a damaged site and by coupling the energy derived from ATP hydrolysis, alter the DNA conformation around the damage site to one that is permissive for endonucleolytic events. The conformational changes may take the form of DNA unwinding.