The export signal has been assumed to be localized not only in the signal peptide of a secreted protein precursor, but also in the N-terminal region of the mature polypeptide chain. Mutant alkaline phosphatases with amino acid substitutions of two positively charged residues (Lys or Arg) in this region at different distances from the signal peptide have been studied to test this assumption. The efficiency of secretion has been shown to decrease in mutant proteins with amino acid substitutions in the region of 16-18 amino acid residues; the closer to the signal peptide is the substitution, the greater is the decrease. A change in the primary structure of the N-terminal domain results also in an increase in the Michaelis constant, which is greater the farther is the amino acid substitution from the signal peptide, suggesting a change in the enzyme function as well.