OmpA protein, a major outer membrane protein of Escherichia coli, is synthesized from a messenger RNA containing a 134-nucleotide 5′ leader region. The role of this leader region in efficient ompA expression was investigated using a series of ompA- lacZ fusion plasmids. These plasmids differ in the amount of DNA encoding the ompA leader region which is fused to the lacZ structural gene. The fusion containing all but six nucleotides of the ompA leader produced the highest β-galactosidase activity, while the fusion containing the shortest leader synthesized only 4% as much β-galactosidase. Fusions with leaders intermediate in length produced between 6% and 24% of the activity found in the most efficient fusion. Quantitaten of lacZ mRNA synthesis by DNA-RNA hybridization revealed differences in lacZ mRNA production reflecting the observed differences in β-galactosidase activity. The primary effect of the ompA leader in maintaining high levels of mRNA is discussed in terms of the roles of mRNA secondary structure.