Site-specific integrative recombination in bacteriophage lambda involves unequal partners. The minimal phage att site is composed of approximately 240 base pairs and has four distinct Int binding sites that differ in size and response to heparin challenge. There appear to be two size classes of Int binding sites, approximately 30-35 base pairs and 15 base pairs. The sites at the common core and in the P' arm are of the former class. Two sites in the P arm are of the latter class. Thus far, three of the four sites have been shown to be necessary for att site function. In contrast, the minimal sequence required for a phage att site partner (such as the bacterial att site) may not be much larger than the 15 base pair common core. We have suggested a model in which integrative recombination involves two unequal partners; accordingly the phage att site is referred to as the "donor" and the bacterial att site, or its analogue, is referred to as the "recipient."