Membranes from acholeplasma laidlawii have been extracted with neutral detergents, which solubilize the proteins and lipids selectively, or with sodium deoxycholate, which gives an almost total solubilization. The amounts of individual proteins present in the detergent extracts of membranes with induced variations in lipid compositions were determined by crossed immunoelectrophoresis. Extraction with the neutral detergent Tween 20 showed that ionic lipids and the glucolipid diglucosyl diglyceride were enriched in the Tween extracts whereas the glucolipid monoglucosyl diglyceride (which cannot easily by accommodated in micelles for geometrical reasons) was enriched in the membrane residue. The amount of monoglucosyl diglyceride in the Tween 20 extracts increased when the content of this lipid was increased in the membrane, and protein D12 was also more easily extracted from membranes rich in monoglucosyl diglyceride. This was not correlated with an increase in the total amounts of D12 in the membrane (as analyzed by crossed immunoelectrophoresis after sodium deoxycholate solubilization), indicating that monoglucosyl diglyceride is involved in the anchoring of protein D12 in the membrane. The total amount of the flavoprotein T4a in the membrane was found to increase upon enhanced amounts of ionic membrane lipids. Furthermore, protein T4a was found to be increasingly extractable upon an increase in the amounts of unsaturated fatty acyl chains in the lipids. Several other proteins also displayed lipid-dependent extractabilities. These results support the hypothesis that for membrane proteins the extractability with neutral detergents and thus interactions with lipids are partly dependent upon the molecular shapes of the membrane lipid molecules. Thus, by use of these selective extraction procedures, information about protein-lipid interactions in the membrane was gained.