Acholeplasma laidlawii B is a simple procaryotic microorganism, without a cell wall, whose membrane lipid fatty acid composition and cholesterol content can be dramatically modified. It is thus possible to markedly vary the temperature and cooperativity of the gel to lipid-crystalline phase transition, thereby altering the phase state and "fluidity" of the A. laidlawii membrane lipids. By varying the chain length of the exogenous fatty acids supplied, it is also possible to modify the thickness of the membrane lipid bilayer. Acholeplasma laidlawii B cell growth and most membrane functions are strongly dependent on the phase state of the membrane lipids. When gel-state lipid predominates at physiological temperatures, cell growth declines sharply, nonelectrolyte permeability exhibits a pronounced local maximum, net glucose transport is markedly reduced, and ATPase activity declines moderately. Upon complete conversion of the lipid to the gel state, cell growth ceases, passive permeability is markedly reduced, and net glucose transport is abolished, but appreciable ATPase activity remains. Provided that the membrane lipid is predominantly or exclusively in the liquid-crystalline state, cell growth and ATPase activity are almost independent of membrane lipid fatty acid composition and cholesterol content, except that cell growth is inhibited by the presence of "hyperfluid" lipid. In contrast, passive permeability and active glucose uptake are influenced by alterations in membrane lipid composition, with increases in lipid fluidity resulting in higher nonelectrolyte translocation rates. Alterations in the length of the membrane lipid hydrocarbon chains per se also appear to affect A. laidlawii growth and membrane function. Thus lipids containing fatty acids with effective chain lengths of 13 carbons or less do not support cell growth or normal enzymatic and transport activities, and lipids with effective hydrocarbon chain lengths of 19 carbons or more do not support normal cell growth.