The kinetics of incorporation of proteins into phospholipid liposomes were studied using fluorescence of the protein tryptophan residue; fluorescence was detd. at varying protein/lipid ratio and at varying temp. Cytochrome b5, purified from liver microsomes, was exposed to unilamellar dimyristoyllecithin liposomes in a fluorescence cuvette. Tryptophan fluorescence of cytochrome b5 increased rapidly as the protein was incorporated into the lipid bilayer. Protein incorporation was biphasic at low lipid/protein ratios, with an initial fast non-exponential phase followed by a slow phase. The fluorescence was detd. at 15, 25, and 35 Deg, and the rigidity of the liposome bilayer was studied as a function of temp. At low temp (15 Deg), protein incorporation was very slow; however, as the lipid phase transition temp. was approached, protein incorporation increased considerably faster. Thus, there is a pos. correlation between liposome fluidity and the ease of protein incorporation into the bilayer. A model is presented. [on SciFinder (R)]