The purpose of the presented work was to study the possibility of using the fluorescent probes method to diagnose the harmful effects of chemical factors on the example of polyethylene glycol on the organism of white rats by evaluating the state of erythrocyte membranes. Material and methods. We used the following fluorescent probes in the studies: ortho-hydroxy derivatives of 2.5-diaryl-1.3-oxazole. Results and discussion. In the case of erythrocytes of rats, which are toxic to PEG-400, there is a marked decrease in the fluorescence intensity of all the probes used. The discussed decrease in the fluorescence intensity of the probes indicates a decrease in the number of molecules of each of the probes associated with erythrocyte membranes per one hour of incubation, which indicates a decrease in the rate of binding of probes to membranes. Such a decrease in the binding rate can be explained by the formation of an additional protective membrane around each lipid membrane. Conclusion. The established fact of formation of an additional coat of polyethylene glycol molecules on the surface of erythrocyte membranes can be treated as a stable standardized indicator for the method of fluorescent probes, which may indicate the absence of damaging effect on the membranes at the object of study, which requires confirmation in further studies.