Growing cells of sterol-requiring Mycoplasma hominis and sterol non-requiring Acholeplasma laidlawii were used to test the ability of cholesterol-dipalmitoyl phosphatidylcholine dispersions to serve as cholesterol donors to these organisms. Dispersions with high cholesterol to phosphatidylcholine ratios were more effective than dispersions with low cholesterol to phosphatidylcholine ratios in donating cholesterol to the membranes of both mycoplasmas and in promoting growth of the sterol-requiring species. M. hominis took up almost three times as much cholesterol as did A. laidlawii. In addition, significant quantities of the phosphatidylcholine component of the dispersions were found to be associated with M. hominis membranes as against none in the A. laidlawii membrane preparations. In all cases, the percentage of cholesterol taken up by M. hominis from the dispersions exceeded that of phosphatidylcholine by a factor of 3-5. These results were interpreted to suggest that all the cholesterol taken up by A. laidlawii is transferred from the dispersion to the membranes by a process which involves only a transient contact between the organisms and the lipid dispersions, whereas a certain amount of the cholesterol taken up by M. hominis may also be derived from lipid dispersions adhering to or fusing with the cell membranes.