Bacteriophage lambda int gene is required for the integration of viral DNA into the chromosome of Escherichia coli. We have extensively purified the product of the int gene (Int) from a lysogen of E. coli that constitutively expresses this gene. Int was assayed by its ability to promote integrative recombination of supertwisted substrate DNA in vitro using a new method based on filter trapping of a recombinant product DNA. In order to catalyze integrative recombination, Int must be supplemented by other factors that can be extracted from bacterial host cells. By itself, purified Int does not demonstrate detectable endonuclease, exonuclease, or nicking-closing activities. However, Int does make stable complexes with double-stranded lambda-DNA containing an attachment site, the region at which recombination takes place. No stable complexes are observed between Int and lambda-DNA without an attachment site or between Int and DNA containing the bacterial site of integration. Int, therefore, appears to be a specificity element that relies on additional factor(s) to provide or activate the catalytic functions required for recombination.