The specific DNA-binding protein integration host factor (IHF) of Escherichia coli stimulates the site-specific recombination reaction between the attP site of bacteriophage HP1 and the attB site of its host, Haemophilus influenzae, in vitro and also appears to regulate the expression of HP1 integrase. IHF interacts specifically with DNA segments containing the att sites and the integrase regulatory region, as judged by IHF-dependent retardation of relevant DNA fragments during gel electrophoresis. The locations of the protein-binding sites were identified by DNase I protection experiments. Three sites in the HP1 attP region bound IHF, two binding sites were present in the vicinity of the attB region, and one region containing three partially overlapping sites was present in the HP1 integrase regulatory segment. The binding sites defined in these experiments all contained sequences which matched the consensus IHF binding sequences first identified in the lambda attP region. An activity which stimulated the HP1 site-specific integration reaction was found in extracts of H. influenzae, suggesting that an IHF-like protein is present in this organism.