The mycoplasmas, a trivial name used to denote organisms included in the class Mollicutes, are a group of prokaryotic organisms comprising more than 120 species distinguished from ordinary bacteria by their small size and the total lack of cell walls. The absence of a cell wall in mycoplasmas is a characteristic of outstanding importance to which the mycoplasmas owe many of their pecu­ liarities, for example, their morphological instability, osmotic sensitivity, unique ion pumping systems, resistance to antibiotics that interfere with cell wall bio­ synthesis, and susceptibility to lysis by detergents and alcohols. The fact that the mycoplasma cells contain only one membrane type, the plasma membrane, constitutes one of their most useful properties for membrane studies; once the membrane is isolated, it is uncontaminated with other mem­ brane types. Another advantage in using mycoplasmas as models for membrane studies stems from the fact that their membrane lipid composition can be altered in a controlled manner. This characteristic results from the partial or total inabili­ ty of the mycoplasmas to synthesize long-chain fatty acids and cholesterol, making mycoplasmas dependent on the supply of fatty acids from the growth medium. The ability to introduce controlled alterations in the fatty acid composi­ tion and cholesterol content of mycoplasma membranes has been utilized in studying the molecular organization and physical properties of biological mem­ branes.