The role of RNA sequences in the 5' leader region between the cap site and initiating AUG in mediating translation was examined in vitro. Hybrid mRNAs were synthesized in which the cognate leader sequence was replaced with either optimized or compromised leader sequences, and translational efficiency was measured for six different coding regions. Translation was most efficient with a leader containing the 5' untranslated region from Xenopus beta-globin and an optimized initiation sequence. Compared with the cognate leaders, this hybrid was observed to increase translation of the various coding regions as much as 300-fold. The translational efficiencies of the different coding regions also varied substantially. In contrast to earlier suggestions that increased leader efficiency results from higher affinity of the leader for a limiting factor, our experiments suggest that increased translation from the beta-globin hybrid leader sequence results from more rapid initiation of translation.