Transduction for lactose-fermenting ability between the lactose-positive lysogen Streptococcus lactis LM0221 and plasmid-cured, prophage-cured, lactose-negative S. lactis LM2301 resulted in the appearance of lactose-positive transductants surrounded by a zone of clearing in the lactose-negative cell lawn. By using plaque assay procedures, the zones were shown to contain bacteriophage particles, and both spontaneous release and UV induction of prophage from these transductants were demonstrated. The DNA hybridization confirmed that LM2301 did not contain the prophage in its chromosome and that the zone-producing transductant KZ1 was relysogenized by the temperate bacteriophage. Further, a 4.35 Kb EcoRI digestion fragment appeared to contain the DNA sequences for integration into the chromosome and may provide a means for stabilizing cloned DNA by effecting chromosomal insertion in LM2301 derivatives. The selection of zone-producing lactose-positive transductants of LM2301 provided a means for detecting strains relysogenized by the temperate phage induced from LM0221.