This study evaluates polyamidoamine PAMAM "starburst" dendrimers (generation 3, Mr 6909) as a potential delivery vehicle for oligonucleotides. Complexes between dendrimer and phosphorothioate oligonucleotides were observed by agarose gel electrophoresis and were positive, negative, or neutral in charge depending on stoichiometry. Complexes were stable in 50% serum to variations in pH (3, 5, and 10) and ionic strength (0-500 mM). Ultrafiltration and gel filtration characterization indicated that the dendrimer:oligonucleotide complexes were primarily < 100 kD, although some larger complexes were formed at oligonucleotide excess. Use of dendrimers resulted in a 50-fold enhancement in cell uptake of oligonucleotide as determined by flow cytometry, and enhanced cytosolic and nuclear availability, as shown by confocal microscopy. These data support the further evaluation of dendrimers for oligonucleotide delivery in cell culture and in vivo.