The number of l-anilino-8-naphthalene sulphonate (ANS) molecules bound to black phospha­ tidylcholine (PC) and phosphatidylinositol (PI)-membranes was calculated. The fluorescence change of membrane-bound ANS was measured after the addition of positively charged proteins to the same side as ANS. Cytodirome c caused a fluorescence decrease, lysozyme and protam ine an increase. These effects were completely reversible in the case of cytodirome c and lysozyme and only partly reversible in the case

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... of protam ine by increasing the ionic strength. The fluorescence polarization of membrane bound ANS was not significantly changed by protein addition. The results are dis­ cussed with respect to the binding of proteins to probe compared with other fluorescent probes.