A set of plasmids carrying different initiator codons -either AUG, or GUG, or UUG, or CUG (as a control) in the hybrid gene IucIZ -was constructed by using synthetic oligonucleotides. GUG and UUG codons were demonstrated to be 2-3-times less effective than AUG in translation initiation. Furthermore, the correlation between the efficiencies of different initiator codons in translation initiation proved to vary, depending on the phase of bacterial growth. The rarely occurring usage in nature of the initiator codons GUG and UUG is supposed to be due to the particular role played by the initiator triplets in regulation of gene expression.