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Polyethylenimine (PEI) is a cationic polymer that is an efficient transfection reagent marred by high toxicity and a susceptibility to aggregate in the presence of serum. Dextran is a biodegradable natural polysaccharide that can be used to reduce the toxicity of PEI and increase its stability in the presence of serum. In this study, small branched PEI units (800/2000 Da) were attached to dextran (Dex; 15/100–200 kDa) to form dextran–polyethylenimine (Dex–PEI) conjugates. The Dex–PEI conjugates were then tested as a gene carrier in the model HEK293 cell line. Dex–PEI conjugates displayed significantly lower cytotoxicity than PEI (25k). Both Dex–PEI and PEI efficiently delivered firefly luciferase encoded plasmid DNA (pDNA) to the HEK293 cells. Dex–PEI resulted in moderately lower transfection efficiencies than PEI 25k when the transfection was carried out in media without serum for 4 h. However, in the presence of serum, which more accurately predicts the anticipated environment of non-viral vectors in vivo, Dex–PEI and unmodified PEI generated similar transfection efficiencies when incubated with the cells for 4 h. When the incubation time of the vectors was increased to 48 h, significantly higher transfection efficiencies were generated by Dex–PEI in comparison to PEI. Turbidity measurements showed that complexes formed between plasmid DNA and unmodified PEI were more susceptible to aggregation in serum-containing media than complexes formed from pDNA and Dex–PEI. Dex–PEI conjugates are therefore believed to have greater potential for translational applications because of lower cytotoxicity characteristics and improved stability in serum containing environments.