Human liver apolipoprotein AI and A II poly(A+) mRNA has been translated in the cell-free rabbit reticulocyte lysate system. The structures of the two primary translation products of these two main protein components of human serum high-density lipoprotein (HDL) have been characterized. The products of the synthesis in vitro are preproapolipoproteins. The signal sequence (pre-sequence) of the primary translation product of human apo AI mRNA consists of 18 amino acids, that of apo AII of 17 amino acids. The cotranslational translocation into dog microsomal vesicles is associated with the cleavage of these sequences by the signal peptidase releasing the proapolipoproteins AI and AII, both extended by an N-terminal hexapeptide. Preproapolipoprotein AII is synthesized in its monomeric form consisting of 100 amino acids. Pro-apo AII is present in the vesicles of the endoplasmic reticulum also as monomer. Sequencing of the radiolabelled signal sequences of both pre-forms revealed their strongly hydrophobic nature. Despite the high affinity of HDL-apolipoproteins for complex lipids their secretion requires these hydrophobic signal sequences for translocation. Internal recognition sequences in the native apoproteins are not responsible for the transmembrane transport.