We confirm the hypothesis of Low (1973) that many E. coli K12 strains contain a prophage (the Rac prophage) located a few minutes clockwise of the trp operon on the genetic map. We have used restriction endonucleases and 32P-labelled probes to construct a physical map of this prophage. Some E. coli K12 strains, including AB1157, have lost the entire prophage, apparently by a specific deletion. This is consistent with prophage excision by site-specific recombination. lambda reverse (lambda rev) phages (Zissler et al., 1971) are recombination proficient derivatives of phage lambda in which the phage recombination functions have been replaced by analogous functions (RecE) derived from the host chromosome (Gottesman et al., 1974; Gillen et al., 1977). Our data support the origin of lambda rev plages by recombination between lambda and the Rac prophage following excision of the Rac prophage from the E. coli chromosome. Important experimental data are included in the Figure legends.