The product of the recA gene of E. coli has been identified by labeling proteins synthesized in UV-treated cells after infection with specialized transducing phages carrying the recA gene. Following infection of UV-treated cells by lambda precA, which carries the recA+ gene, a major protein with a molecular weight of 43,000 is detected on polyacrylamide gels containing sodium dodecyl sulfate. This protein is also made after infection of suppressing hosts by lambda precA99, which carries an amber recA- mutation, but is not synthesized after infection of nonsuppressing hosts by this transducing phage. A spontaneous recatrevertant of lambda preca99 induces synthesis of this protein after infection of a nonsuppressing host. The product of the recA gene is a soluble protein found in a complex with a molecular weight of approximately 150,000 after mild detergent lysis of cells.