Stimulated and resting mononuclear leukocytes were incubated with a stilbazolium merocyanine dye 1-(6'-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5- dienylidene)ethylidene]-1,4-dihydropyridine and immobilized in isotropic and stretched polyvinyl alcohol film. Polarized absorption, fluorescence and fluorescence excitation spectra were collected and the anisotropy of absorption and emission were calculated. Analysis of the spectra pointed to: i. the occurrence of perturbation of the membrane structure by incubation with the dye, and ii. influence of the blood serum addition, during the process of incubation with the dye, on the efficiency of incorporation of merocyanine into the cells and the degree of the dye orientation in the membrane. A small fraction of the dye molecules introduced into resting cells was found oriented to a higher degree than a large fraction incorporated into stimulated cells. The incubation time longer than 15 min caused strong changes in the membrane structure both of the resting and stimulated cells.