Abstract: Inctose－negative mutants were derived from streptococcus lactis 6030 and 6025 treated by ultravi－olet irradiation and ethidium bromide． The nature of the carbohydrates fermention and the β－ galactase activity were determined in the mutants． The β－galactase genes of S． Lactis 6030， 6025 and Lacmutants were analyzed by a DIG-β-galactase gene probe．The results showed that the lactose metabolism of S·Lactis was associated with the presence of the plasmid．

Key words: Streptococcus lactis； Lac-mutant； &beta, －galactase；