The data presented in this report show that the microviscosity of the plasma membrane and the cholesterol/phospholipid molar ratio of its lipid bilayer increases during ageing of Acholeplasma laidawii cultures. At the same time the age changes of other lipid components content do not correlate with the change of membrane viscosity. It is also shown that membrane enzyme activities of mycoplasma cell decrease with age. These results confirm the membrane hypothesis of ageing according to which increased microviscosity is the essential factor of cell ageing.