Polyethyleneimine (PEI) is shown to destabilize isolated rat liver lysosomes, as indicated by a decrease in the latency of their acid N-acetyl-beta-glucosaminidase. PEI also inhibited the generation of radiolabeled digestion products from 125I-labeled bovine serum albumin endocytosed by rat visceral yolk sac in vitro. However, PEI did not greatly inhibit the endocytic uptake of a nondigestible fluid-phase substrate, fluorescein isothiocyanate (FITC)-dextran. It is hypothesized that PEI inhibits the adsorptive endocytosis of 125I-labeled bovine serum albumin, and thus its subsequent intralysosomal digestion, by competing with and displacing the labeled protein from its binding sites on the visceral yolk sac cell surface. This hypothesis suggests a plausible explanation for the ability of PEI to act as an efficient vector for gene and oligonucleotide transfer into mammalian cells. PEI present in the culture medium is carried into cells by adsorptive endocytosis. Concentrated thus on the endosome membrane, it permeabilizes this membrane and so affords DNA conjugated to the PEI an otherwise unavailable mode of access into the cytoplasm.