Lipid-lipid interactions during complex formation and after addition of serum. The relative fluorescence intensities of DOTAP and DC liposomes containing both NBD-PE and Rh-PE were determined at 520 nm for five minutes. DOTAP-high, DOTAP-low, DC-high and DC-low complexes were formed by addition of DNA and protamine sulphate at the ratios described in the text while continously monitoring the fluorescence intensities. The change in the fluorescence intensities was determined after addition of serum and Triton-X 100. Values were corrected for the dilution factor experimentally determined and are expressed as percentage of the mean fluorescence intensity after addition of Triton-X 100. The experiment was repeated thrice with similar results.