This study examined the action of anisosmotic media on the volume of nucleated erythrocytes isolated from Rana temporaria. Elevation of medium osmolarity from 100 to 345 mOsm resulted in attenuation of mean cell volume by more than 3-fold, estimated by hematocrit measurement. By contrast to this ‘classic’ erythrocyte volume evaluation technique, we did not observe any significant cell volume modulation by examining the 3D reconstruction of erythrocyte interference images obtained by laser interference microscopy. Comparative analysis of mean cell volume, phase height and cell area appraised by laser interference microscopy showed that the lack of visible alterations of phase image geometry was caused by sharp elevation of the average refractive index of the cytoplasm in shrunken cells. Thus, our results show for the first time that laser interference microscopy in combination with a direct method for cell volume measurement may be employed for estimation of the refractory index of intracellular milieu and for assessment of changes of physical chemical properties of the cytoplasm evoked by diverse stimuli including osmotic stress.