It was determined that, ultraviolet- B (UV-B) radiation (280-320) induces the structure changes in the lipid bilayer and annular lipids of the plasma membranes of yeast cells. The object of the investigation was the yeast cells of Candida guilliermondii CMU-916. For the evaluation the structural state of the membranes, the microviscosity of the lipid phase was determined. The fluorescence of the samples was measured on a spectrophotometer (Fluorescence spectrophotometer Varian Cary Eclipse 2007) at a peak of the excitation light wavelength of 334 nm to evaluate the microviscosity and polarity of the lipid bilayer and annular lipids of yeast cells. The fluorescence peak of excimer pyrene FE was recorded at an emission wavelength of 470 nm, and the fluorescence peak of the Fm monomer at an emission wavelength of 393 nm. The obtained data while using a fluorescent probe pyrene indicate a change in the studied parameters of the structural state of cell membranes. Based on the change in microviscosity of the yeast membranes after irradiation, it can be assumed that a structure modification can lead to a change in the polarity in the lipid bilayer. It has been shown that with increasing UV-B radiation, microviscosity and polarity in the lipid bilayer and annular lipids increase. According to the obtained results, it is assumed that an increase in microviscosity and polarity of cell membrane lipids is associated with the lipid peroxidation (POL). The combination of the obtained data suggests that after irradiation of yeast cells, a change in viscosity characteristics, a decrease in the fluidity of the membrane lipid component, are the reflections of adaptive structural and functional rearrangements. It is known that, during the stimulation of lipid peroxidation in the membranes, decreases the lipid content, microviscosity and electrostatic charge also change.