Using the spin labelling method we studied changes in the structure and dynamics of molecular mobility in the plasmatic membrane accompanied by a spontaneous alteration of a melanotic melanoma line into an amelanotic form with a higher growth rate, changed antigenicity and immunogenicity. The calculated ratio of the low-field line (A) intensity to the central line (C) intensity of the spectrum showed statistically significant differences in the order of parameters in the plasmatic membranes of both forms of melanocytes. The significantly broader central line (deltaW0) in the spectra of labelled amelanotic melanoma cells than in the original cell line indicated changes in the membrane structure leading to a lowering of the degree of order in the phospholipid bilayer. It has been suggested that a progression of transplantable melanomas is accompanied by an increase in membrane fluidity and reduction in molecular mobility dynamics within it.