When placed downstream of the start codon, multimers of the dinucleotide CA stimulated translation from lacZ, gusA and neo mRNAs in the presence or absence of an untranslated leader sequence. Enhanced expression in the absence of a leader and Shine-Dalgarno sequence indicated that stimulation by CA multimers was independent of translation signals contained within the untranslated leader. Multimers of CA stimulated a significantly higher level of lacZ expression than multimers of individual C or A nucleotides. Translation levels increased as the number of CA repeats increased; fewer multimers were required for enhanced expression from leadered mRNA than from mRNA that was deleted for its leader sequence. Addition of down-stream CA multimers increased the ribosome binding strength of mRNA in vitro and the amount of full-length mRNA in vivo, suggesting that the enhanced expression resulted from translation of a more abundant functional message containing a stronger ribosome binding site. The presence of downstream CA-rich sequences, occurring naturally in several Escherichia coli genes, might contribute to translation of other mRNAs. Addition of CA multimers might represent a general mechanism for increasing expression from genes of interest.