HBpep-SA acetate is a peptide aggregate that is permeable to cell membranes and capable of phase separation to form stable microdroplets when the pH is below 6.5. This compound efficiently delivers a wide range of macromolecules, from small therapeutic peptides of 726 Da to large enzymes up to 430 kDa. The loaded aggregates remain stable under near-physiological and serum conditions until internalized by cells. HBpep-SA acetate is also employed for intracellular delivery of large macromolecules and as a carrier for mRNA-based vaccines.