Abstract: Lipophilic fluorescein-based molecules have been synthesized and incorporated into thin plasticized polymeric membrane films as chromoionophores for the optical sensing and detection of polycationic protamine. The membrane response is based on the extraction of protamine into the film due to the interaction between the anionic fluorescein and the protamine polycation which results in a cation exchange between the protamine and proton, and thus, induces an absorbance spectra change of the polymeric film. The response speed is controlled by protamine diffusion through the stagnant diffusion layer adjacent to the film surface as well as within the bulk of the polymer film. When limited exposure time and non-stirring detection modes are used in a buffer solution, absorbance of a film changes as a function of the protamine concentration in the range of about 2 to 60 .mu.g/ml (0.44 to 13.3 .mu.M).