The gam locus of bacteriophage λ encompasses two coding sequences with the same reading frame and translational stop, one corresponding to an M r 11646 polypeptide ( gamS gene), the other to an M r 16349 polypeptide ( gamL gene). A DNA segment encoding gamS but not gamL was placed under π R promoter control (regulated by the cIts857-coded repressor) on a multicopy plasmid, and an insertion mutation ( gamS201) was constructed. Expression of gamS +, but not gamS201, inhibited Escherichia coli RecBC nuclease in vivo; the criteria were inhibition of chromosomal DNA degradation after UV irradiation and plating of T4 gene 2 − phages. The recB + C + bacteria expressing gamS + were completely or partially similar to recC − mutants with respect to certain phenotypes: defective plating of phages P1 and P2, ability to plate (in a recA − background) λred − gam − phages, reduced resistance to UV irradiation, defective SOS induction, decreased colony-forming ability.