Complexation of DNA to cationic liposomes composed of DC-Chol:DOPE has been developed to deliver the CFTRcDNA gene to the pulmonary epithelium of cystic fibrosis patients. The target of drug action is the bronchiolar epithelium, consequently the complex may be delivered by jet-nebulisation. Few studies have been undertaken to assess the significance of nebulisation on the transfection efficiency of the complex.A cell culture flask was adapted to allow the collection of nebulised complex directly onto cultured cells or into aqueous phase prior to physicochemical analysis. Different total concentrations of reporter gene (pCMV-β) and liposomes (1:5 weight ratio), were nebulised with an Acorn system 22 nebuliser operated at 6 l/min. Zeta potential (Zeta sizer 3), size (Coulter N4) and gene transfer efficiency were measured before and after nebulisation.There was no effect of nebulisation on the Zeta potential or transfection efficiency at low concentrations of complex (20μg DNA/5ml). There was a significant decrease in transfection efficiency at the intermediate concentration (60μg DNA/5ml) of 68.2% of the control i.e. instillation of complex, but no effect on Zeta potential. At high concentrations of complex (120μg DNA/5ml) there was a significant decrease in Zeta potential from -16.4mV (+/-0.8) to -26mV(+/-3.5), and there was no transfection following nebulisation. At each concentration of complex there was a significant decrease in size after nebulisation from 387.4(+/-9.5) to 270.0(+/-11.4)nm.From the observed reduction in complex size, it appears that the complex may be sheared by the process of nebulisation. At high concentrations of complex this shearing process has a dramatic effect on the Zeta potential of the surface and consequent transfection efficiency. Therefore care must be taken in designing human gene therapy trials delivering DNA to the lung. High concentrations of complex will probably be required to treat the lung, and therefore a low dose of complex delivered over a long period may be necessary to ensure optimum efficacy.