Summary: The induced formation of β-galactosidase in relation to overall protein synthesis in Escherichia coli was examined under conditions where the enzyme inducer was required for growth. There was, initially, a substantial production of β-galactosidase without any detectable increase in total cell protein. This phenomenon has been termed ‘preferential’ β-galactosidase synthesis. The occurrence of preferential β-galactosidase synthesis indicates that β-galactosidase formation is a discrete event in cellular metabolism, and is not necessarily geared to overall cell growth. The data obtained also suggest that the carbon source(s) for preferential β-galactosidase synthesis might be of endogenous origin.