By analyzing the effects of single base substitutions around the ATG initiator codon in a cloned preproinsulin gene, I have identified ACCATGG as the optimal sequence for initiation by eukaryotic ribosomes. Mutations within that sequence modulate the yield of proinsulin over a 20-fold range. A purine in position -3 (i.e., 3 nucleotides upstream from the ATG codon) has a dominant effect; when a pyrimidine replaces the purine in position -3, translation becomes more sensitive to changes in positions -1, -2, and +4. Single base substitutions around an upstream, out-of-frame ATG codon affect the efficiency with which it acts as a barrier to initiating at the downstream start site for preproinsulin. The optimal sequence for initiation defined by mutagenesis is identical to the consensus sequence that emerged previously from surveys of translational start sites in eukaryotic mRNAs. The mechanism by which nucleotides flanking the ATG codon might exert their effect is discussed.