Sub-inhibitory concentrations of lincomycin slightly inhibit growth of Escherichia coli carrying plasmid RP4 and cause a 2-fold increase in TEM-2 β-lactamase. To analyze this effect, cultures were pulse-labeled with [³H]leucine, chased with non-radioactive leucine and immunoprecipitated with anti-β-lactamase antiserum. The synthesis rate of β-lactamase was two times higher in inhibited cultures than in control cultures. No significant decrease of labeled enzyme occurred during the 30 minutes chase, indicating no degradation of β-lactamase. The rate of maturation of pre-β-lactamase was determined by measuring the decrease in the amount of pre-β-lactamase after a 1-minute labeling interval. There was no significant difference between the control and lincomycin-treated cultures, indicating that posttranslational translocation is not involved in the stimulation. Both plasmid encoded and chromosomally encoded TEM-1 β-lactamase increased in the presence of lincomycin. The effects of other protein synthesis inhibitors on the synthesis of TEM-1 β-lactamase were examined. The stimulation of β-lactamase synthesis by lincomycin appears to be specific for macrolide and related antibiotics and is not a general phenomenon resulting from partial inhibition of protein synthesis.