Most mycoplasmologists support the proposal of Gibbons and Murray (1978) endowing the mycoplasmas with the lofty status of one of the four major divisions in the kindgom Procaryotae, the division named Mollicutes (mollis soft + cutes skin) to denote the lack of cell walls in these organisms. The recent report that penicillin-binding proteins and enzymes of peptidoglycan synthesis are absent from mycoplasmas as against their presence in the plasma membrane of the wall-less bacterial L-forms (Martin et al., 1980) supports the idea of a separate division for mycoplasmas. A different view has recently been expressed by Fox et al. (1980) who, on the basis of nucleotide sequences in 16 S rRNA, argue that mycoplasmas are genealogically wall-less descendents of clostridia, and consequently should not be given a separate high taxonomic status. Notwithstanding this somewhat philosophical controversy, the fact is that mycoplasmas differ from other prokaryotes in several unique properties, most useful in membrane studies. Thus, the mycoplasmas are unique in being the only self-replicating organisms with a single membranous structure—the plasma membrane. This is, perhaps, their greatest advantage in membrane studies, for it facilitates the isolation of pure plasma membranes uncontaminated by other membrane types. Moreover, the mycoplasmas’ lack of cell walls enables the application of gentle and simple techniques, such as osmotic lysis for membrane isolation (Razin, 1978, 1981).