Escherichia coil lacZ is a frequently employed reporter gene for the monitoring of gene expression and recombinant protein production due the simple determination of beta -galactosidase activity in both qualitative and quantitative assays. In the absence of either total or recombinant protein synthesis, we observed a lack of correlation between protein amount and enzymatic activity in both engineered and native beta -galactosidases in Escherichia coli cells. A delayed fading of beta -galactosidase activity compared with the rapid degradation of intact protein suggests a progressive increase in enzyme-specific activity during the life of the protein. This intriguing event does not involve solubilization from major protein aggregates and it occurs both in vivo and in cell extracts, but not in solutions of purified protein. Possible explanations for this activation are examined in the context of the assisted protein folding network and proteolytic degradation of misfolded proteins. (C) 2001 John Wiley & Sons, Inc.