F′ pro + plasmids were selected and used as donors to prepare P22 transducing phages. Two types of result were observed. pro + from type I donors cannot be packaged by wild-type P22 to yield transducing particles unless a prophage pac site is introduced into the plasmid. Transposon Tn10 also allows initiation of packaging. pro + from type II plasmids can be transduced with the same efficiency as pro + DNA on the chromosome, indicating that a chromosomal pac site was included when the F′ pro + was excised from the Hfr strain. The usefulness of type I plasmids as a test substrate for pac signals is discussed.