The effect of substitutions for the positively charged Lys(-20) in the N-terminal domain of the E. coli alkaline phosphatase signal peptide on enzyme secretion has been studied. Mutant alkaline phosphatases were obtained by the amber-suppressor method. An amber mutation was introduced in the appropriate position of the alkaline phosphatase gene using oligonucleotide-directed mutagenesis. This was followed by mutant protein synthesis in E. coli strains producing amber-suppressor tRNAs specific for Tyr, Gly, Ala, Glu, Phe, His, Cys, and Pro. All the mutant proteins can by translocated through the cytoplasmic membrane and form in the periplasm a molecule possessing an enzymatic activity. However, some amino acid substitutions decrease the rate of protein maturation their effect depends not only on the charge of the amino acid residue but also on its nature. Thus, introduction of positively charged. His and the polar uncharged Tyr is without effect, while negatively charged Glu and hydrophobic Ala, Phe and Pro residues as well as Gly and Cys have an inhibiting action. The results obtained testify to the importance of the signal peptide terminal domain primary structure in secretion.