The phospholipids of both mitochondrial and microsomal membranes from normal liver, host liver, and Morris hepatoma 7777 were isolated, separated, and quantitated. The total as well as the individual fatty acid concentrations and compositions were determined. The total phosphlipids isolated from tumor mitochondria were idly altered, compared with mitochondria from other normal or host liver. The polyenoic acids were decreased, and there was a concomitant increase in the monoenes. When the respiratory control was determined, the tumor mitochondria exhibited a significant decrease in this parameter. The tumor microsomal membrane fraction, on the other hand, contained about 50% less phospholipid than the controls. The fatty acid patterns of the total as well as the individual phospholipids were quite similar to those observed in the mitochondria. The species of phosphatidylcholine from both membrane fractions were separated by argentation chromatography of the intact molecules, and, as predicted by the fatty acid compositions, the major species of the tumor was the monoenoic/dienoic fraction. The acyl coenzyme A:1-acyl glycerophosphorylcholine acyltransferases, which aid in controlling the fatty acid composition of phospholipids, were measured. The very marked increase in activity of these enzymes toward polyenoic as well as monoenic fatty acids suggested that the polyenoic acids were not available for use in the resynthesis of the phosphatidylcholines in the tumor.