The accessory recombination function (arf) gene of bacteriophage P22 is located immediately upstream of the essential recombination function (erf) gene. Three mutant alleles of arf were constructed and installed in P22 in place of the wild-type allele: an out-of-frame internal deletion, an in-frame internal deletion, and an amber mutation. The deletion mutant phages are partially defective in homologous recombination and plaque formation in wild-type and recA hosts; their defects are more severe in recB and recA recB hosts. The amber mutant phage exhibits the same growth phenotypes in nonsuppressing hosts, but not in an amber-suppressor host. Plasmids that express arf complement the growth defect of arf- phages. These plasmids stimulate erf-mediated recombination; they were also found to cause a small stimulation of recA-recBCD-mediated homologous recombination of phage lambda.