Low-molecular-weight PEIs and cationic liposomes can be combined resulting in a synergistic increase in transfection efficiency as we have reported earlier. Here, we have further investigated the potential mechanisms of this synergy. Complex morphology, complex sizes and DNA condensation were studied using transmission electron microscopy, light scattering methods and ethidium bromide exclusion, respectively. Cellular uptake, transfection efficiency, and effect of proton pump inhibitor bafilomycin A1 were examined in cell cultures. The cellular uptake of DNA was negligible with PEI2K-DNA complexes, whereas the uptake of the PEI2K-DNA-Dosper or the Dosper-DNA complexes was maximally about 40%. The number of transfected cells was two times higher with PEI2K-DNA-Dosper complexes than with Dosper-DNA complexes. The PEI2K-DNA-Dosper combination was slightly less sensitive to bafilomycin A1 than the PEI25K-DNA or Dosper-DNA complexes. There were no differences between PEI2K and PEI25K in DNA condensation. Dosper condensed DNA slightly more in PEI2K complexes. The PEI25K-DNA complexes were much smaller (<250 nm) than the PEI2K-DNA complexes (0.5-12 micro m) which were also rather polydisperse. It is suggested that two independent mechanisms would lead to synergistic transfection efficiency: (1) Dosper improves the cellular uptake of PEI2K-DNA complexes, and (2) PEI2K improves a transfer of the complexes from lysosomes to nucleus.