Tryptophanyl emission spectra of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (G3PDH) were measured after the addition of liposomes prepared of natural phospholipids: phosphatidylinositols (PI), phosphatidylserines (PS) and phosphatidylcholines (PC). The measurings were made for various molar lipid/protein ratios (100-1000). A decrease in the enzyme fluorescence intensity and a "red" shift of the emission band maximum were observed. The susceptibility of the enzyme fluorescence to liposome action strongly depended on the kind of phospholipid and changed in the sequence PI greater than PS greater than PC. The presence of liposomes affected the accessibility of tryptophan residues for the fluorescence quencher (acrylamide). The results suggested that interaction induces some specific conformation changes in the enzyme molecules which may be responsible for modification of the enzyme activity. A comparison of the modification in fluorescence characteristics with those observed during denaturation suggested that the denaturation mechanism is not operative. Other possible mechanisms of the interaction are discussed.